feat(shafts): a drainage tree -- the field goes from 22% to 97% connected

The plateau was the tell. Roughness changed the component COUNT (it
manufactures noise pockets) but the giant component sat at ~0.22 in every
variant, and widening connectors past the proven roughness bound barely
moved it. That is a random graph below its percolation threshold: 0.6
sites, 0.35 bonds, and a 1.6-cell pair cutoff that drops many neighbour
pairs once jitter spreads them 1.7 (orthogonal) to 2.4 (diagonal) cells
apart.

So the lever was topology. Every shaft now emits a deterministic connector
to the nearest shaft STRICTLY closer to the origin, so chains drain inward
and terminate at the spine. The probabilistic connectors stay as texture
and loops -- the tree guarantees you can get out, the noise keeps it from
reading as a diagram.

Deliberately NOT tuned: raising CrossConnectChance/ShaftDensity/the cutoff
toward the percolation threshold buys a world connected at one seed and
shattered at the next, which is exactly the 1-in-16 behaviour we had. A
phase transition is not a knob.

  seed 1337   components 30 -> 17,  largest share .220 -> .975
  16 seeds    spine is the largest component in 16/16 (was 8/16)
              largest share .58 - .9999

Seam safety, because a parent computed from a 3x3 window would differ
between chunks and the connector would exist in one and not the other:
collect over 7x7, emit only for the inner 3x3, whose 5x5 candidate windows
are then fully contained. 154 seam probes per path, 0 mismatches.

Box verdicts re-checked against the new air: 27 proved, 33 mixed,
0 violations. Stack mirror bit-identical over 20,000 samples.
Perf: 1.634 us forced rebuild, 0.100 us hot call -- the 49 rolls happen
only on cache rebuild, never per voxel.

⚠️ ACCEPTANCE STILL FAILS: arrival->departure 4/16 (was 1/16). Reported,
not tuned. With 97% of the air in one component, two mouths that both land
IN the field would essentially always connect -- so the remaining failure
is placement, not topology. Next.

Build clean, 20 succeeded / 0 warnings / 0 failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bp7FxoQcSoiuhxjuLr6aBM

CODEMAP.md

@@ -151,7 +151,7 @@ bit. They are port-correctness oracles, not fidelity checks: the acceptance bar
 | `VoxelDensityOps::MakeSdfFill` | 2 | The same op with `Sign = +1` — FloatingIslands' `Density += Fill·Base·2`. ±1 multiplication is exact in IEEE-754, so the carve path is bit-for-bit unchanged by the generalisation. |
 | `VoxelDensityOps::MakeSlabVoidSource` | 1 | Floor surface + ceiling surface → void field. **XY-pure** since §3.1, which is what gives it an **exact `ClassifyBox` with no sampling**: FBM's `[-1,1]` contract bounds both surfaces into known Z bands. Serves FlatPlain **and** CrystalChamber. |
 | `VoxelDensityOps::MakeGridColumnMod` | 3 | Infinite-height cylinders on a world grid, 3×3 cell memo. Adds solid only ⇒ `FillOnly` when a column reaches the box, `Identity` otherwise — and that `Identity` is what lets the source's `AllAir` verdict survive. |
-| `FShaftFieldSource` (internal) | 1 | VerticalShafts' memoised 3×3 shaft/capsule source. In origin-adjacent query cells it adds the runtime `(0,0)` spine radius as a connector-only endpoint; `VoxelHash::Pair` and the existing cutoff/gate remain deterministic. Spine capsules use a `sup|FBM|=1.5` roughness-safe radius, while the structural post still owns the column and the ledge modifier ignores the synthetic endpoint. `EffectOverBox` mirrors the rule conservatively. |
+| `FShaftFieldSource` (internal) | 1 | VerticalShafts' memoised inner 3×3 shaft/capsule source. Each rebuild collects 7×7 cells so every emitted inner shaft has its complete 5×5 candidate window; the nearest strictly more-central shaft is the deterministic parent, and a local minimum links to the runtime `(0,0)` spine. Existing `VoxelHash::Pair`/`Spacing*1.6` links remain additive texture/loops. Tree and spine capsules use a radius strictly above `sup|FBM|=1.5`; the structural post still owns the column and the ledge modifier ignores the synthetic endpoint. `EffectOverBox` mirrors both tree and random capsules conservatively. |
 | `VoxelDensityOps::BuildSlabStack` | — | 5 ops, **no branch on archetype**: FlatPlain and CrystalChamber differ only in defaults, exactly as `GetSlabDensity` already had it. 8 archetypes → 7. |
 | `FSurfaceColumnSource` (internal) | 1 | The bridge between the two spaces: consumes the ground + sky-cap **height** stacks and produces density. `IsXYPure()` **false** — the heights are XY-pure, a distance to them never is. Owns a **six-box spatial LRU** of direct-indexed per-column cells, keyed by `PrepareChunk` on `(StrateBottomWorldZ, LayoutVersion, Seed, ParamsFingerprint)` so it is **shared down the whole vertical strate stack**, exactly like `GSurfColCache`. Six 81×81 boxes preserve hot columns across interleaved regions at roughly 0.79 MiB TLS before padding (more memory, fewer whole-cache recenter/recompute misses). Fractional XY remains direct-compute. |
 | `VoxelDensityOps::BuildSurfaceStack` | — | SurfaceWorld, complete: column + overhang + 3 structural, plus biome blending when `PerBiomeParams` is non-empty. Takes ownership of an `IVoxelBiomeField`. |
@@ -481,13 +481,13 @@ The plugin's first tests (`OPSTACK-PLAN.md` Phase 0.5). Run them from the editor
 | `VoxelForgeCrossPlatformTest.cpp` | `VoxelForge.Determinism.CrossPlatformDigest` | SHAPE digest (sign of density = the world) + FIELD digest (bit-for-bit) over a fixed integer grid, plus `NearIso` bounding how many samples could flip sign. Reports rather than asserts until pinned. Run on Windows and Linux and compare. |
 | `VoxelForgeOpStackSlabTest.cpp` | `VoxelForge.OpStack.SlabEquivalence` | **Phase 2's first port.** The same 5-op slab stack vs `GetSlabDensity` over 20k points, run twice — FlatPlain **and** CrystalChamber — which is what demonstrates the two archetypes really are one op. Plus window-invariance and box-verdict brute force. Compares against the reference **as it is now** (post Z-term removal), so green = pure refactor and any visual delta is attributable to §3.1 alone. |
 | `VoxelForgeOpStackTunnelTest.cpp` | `VoxelForge.OpStack.TunnelNetworkSpineEquivalence` | **Stage A of the last port.** Zeroes the 13 detail-op amplitudes so the *original* takes the path stage A ported — that is what makes an incomplete stack verifiable now. Samples in **clusters** (24 chunks × 250 points), because the SDF cache rebuilds when a query leaves its box and uniform sampling would rebuild per point on both paths. Check 3 (two param sets, A/B interleaved) compares each stack **to itself alone, never to the original** — the original would fail it, see AUDIT §C2. Asserts **zero** box verdicts, which is the honest stage-A result. |
-| `VoxelForgeOpStackShaftTest.cpp` | `VoxelForge.OpStack.VerticalShaftEquivalence` | The port that tests **reuse**, not fidelity: three of the five ops are Maze's, unchanged. Forces connectors + ledges on, because both are off or negligible at defaults and a resting param is an untested operator. **Fixed 2026-07-29:** its `EffectOverBox` used to return `CarveOnly` because a shaft merely *existed* within a `Spacing*1.6` halo — true almost everywhere at `ShaftSpacing 55 / ShaftDensity 0.6`, hence **0 of 60** tiles. It now rebuilds the connectors the way `GetCells` does (same row-major cell order ⇒ same `VoxelHash::Pair`, so symmetry of `Pair()` is not assumed; sweeping `[box cells] ± 1` is a superset of any 3×3's pairs) and tests the real capsule, with **Z exact** (horizontal capsule at `Zc`) and XY conservative. The sampler was also widened from ±48 voxels to ±440 — it was under one `ShaftSpacing`, the same trap as the tunnel test's ±32-vs-80. Every proved tile is brute-forced over its full lattice. |
+| `VoxelForgeOpStackShaftTest.cpp` | `VoxelForge.OpStack.VerticalShaftEquivalence` | The port that tests **reuse**, not fidelity: three of the five ops are Maze's, unchanged. Forces connectors + ledges on, because both are off or negligible at defaults and a resting param is an untested operator. The source and mirror now share a deterministic inner-3×3-emitted drainage tree whose parents come from rebuild-only 7×7/5×5 windows, plus the old random links. **Fixed 2026-07-29:** its `EffectOverBox` used to return `CarveOnly` because a shaft merely *existed* within a `Spacing*1.6` halo — true almost everywhere at `ShaftSpacing 55 / ShaftDensity 0.6`, hence **0 of 60** tiles. It now tests tree and random capsules (same row-major cell order ⇒ same `VoxelHash::Pair`, so symmetry of `Pair()` is not assumed), with **Z exact** and XY conservative. The sampler was also widened from ±48 voxels to ±440. Every proved tile is brute-forced over its full lattice, and the test reports rebuild-vs-hot cache cost. |
 | `VoxelForgeOpStackIslandTest.cpp` | `VoxelForge.OpStack.FloatingIslandEquivalence` | The port that runs the stack **backwards** — void + fill vs rock + carve, same classes with the opposite sign. Counts interior-solid and open-void samples separately (on this archetype an aggregate "N solid" is dominated by the seal bands and says nothing about the islands). Counts `AllSolid` and `AllAir` verdicts **separately** too: `AllAir` is the one no cave archetype could ever prove, and it is the entire perf argument here. |
 | `VoxelForgeOpStackMazeTest.cpp` | `VoxelForge.OpStack.MazeEquivalence` | **Phase 1's load-bearing test.** The 7-op Maze stack vs `GetMazeDensity` over 20k points (aiming for bit-identity; a side-of-iso disagreement is the hard fail), plus purity across workers and brute force on every box verdict the stack emits. Reports how many tiles the stack can prove uniform — today's `ClassifyTile` proves **zero** for any cave archetype. |
 | `VoxelForgeStrateParamCoverageTest.cpp` | `VoxelForge.Determinism.StrateParamBlendCoverage` | **The X-macro guard** (added 2026-08-17). `FStrateGenerationParams::Lerp` blends the hand-written `VF_STRATE_PARAM_FIELDS` list, **not** the struct — so a field added to one and not the other compiles, tests green, and silently takes its **default** inside every Gradient/Interleaved transition band. This expands the X-macro a **third** way (after LERP and SNAP), into a name list, and diffs it against the struct's UObject reflection. Pure shape test: no fixture, no world, instant. `GExemptFieldNames` is **empty** — every reflected field is covered today, and any exemption must be written down as a decision. Stakes rise with the world composer, which intends to invent parameter sets through this same `Lerp` (`COMPOSER-NOTES.md`). |
 | `VoxelForgeLayoutOrderIndependenceTest.cpp` | `VoxelForge.Determinism.LayoutOrderIndependence` | Builds a known transient soft-pointer pool, then rebuilds it in original, reversed, and swapped orders. Requires a non-empty layout and passage set, and compares every slot's definition/Z/height plus passage endpoints, radius, type, control geometry, and bounds bit-for-bit. |
 | `VoxelForgePassageOpenSpaceTest.cpp` | `VoxelForge.Determinism.PassageLandsInOpenSpace` | Uses the real fixture density path to check every generated inter-strate passage whose destination query answers: a 16-point ring outside the mouth's carve/blend band has at least half its samples in destination air, and the endpoint matches the pure open-point result within the mouth's float envelope. This is a connectivity proxy, not a flood-fill proof. Reports checked passages and false/unanswerable archetypes; fails if it inspects zero passages. |
-| `VoxelForgeStrateConnectivityTest.cpp` | `VoxelForge.Generation.StrateConnectivity` / `VoxelForge.Generation.StrateConnectivityRefinement` | Bounded strate metrics with density-polarity and solid-gap controls, deterministic route rechecks, and refinement sweeps. The refinement test explicitly checks margins 0/2/4/8 at step 2/radius 192, reports the `(0,0)` spine's full-window/interior vertical run and component for all eight fixture strates plus arrival/departure→spine and arrival→departure enums, then repeats those spine/mouth measurements across 16 VerticalShafts seeds and prints the post-fix acceptance count. |
+| `VoxelForgeStrateConnectivityTest.cpp` | `VoxelForge.Generation.StrateConnectivity` / `VoxelForge.Generation.StrateConnectivityRefinement` / `VoxelForge.Generation.VerticalShaftSeamFreedom` | Bounded strate metrics with density-polarity and solid-gap controls, deterministic route rechecks, and refinement sweeps. The refinement test explicitly checks margins 0/2/4/8 at step 2/radius 192, reports components/largest-component share, the `(0,0)` spine's full-window/interior vertical run and component, plus arrival/departure→spine and arrival→departure enums across 16 VerticalShafts seeds. The seam test re-evaluates cell-boundary positions after warming distinct neighbouring chunk contexts in both legacy and operator-stack paths. |
 
 ## 4. The density pipeline (most-edited hot path)