test(measure): VerticalShafts breaks the primordial law in 15 of 16 seeds

The margin is exonerated. Every earlier run excluded 8 voxels at each
strate boundary, and voxels 4-8 of those are ordinary interior, not seal
-- the same window mistake that produced "FlatPlain is 99.95% void" at
margin 32. So the query was re-run at margin 0, 2, 4 and 8. All four stay
negative; margin 0 includes the seal itself and still reports negative.
(The implementation had clamped explicit margins to >= 1, which made the
margin-0 check impossible; Codex found and fixed that.)

Diagnosis -- this is not a near-miss wall:
  arrival   LowerPoint (65.8,  95.8, -1047.25)
  departure UpperPoint (88.4, -104.6,  -991.58)
  XY separation 201.7 voxels (~50 m), Z delta 55.7
  arrival component   22,531 cells (22.0% of air)
  departure component 21,621 cells (21.1% of air)
  nearest approach between the two components: 135-179 voxels

Two large, healthy, entirely separate shaft networks half a strate apart.
Nothing to breach; the archetype has no lateral connection at all.

16-seed sweep: 1 pass, 15 fail. Not one unlucky world -- structural.

So: descend into a VerticalShafts strate and you probably cannot reach
its exit. This is the "live hole in the shipping game" COMPOSER-NOTES
Tier 1 predicted, now measured instead of assumed.

Stated precisely, because it names the fix: VF_SuggestLandingPoint makes
each mouth land in open space, but NOTHING makes the two mouths of one
strate land in the SAME open space.

New: VF_DiagnoseConnectivity returns component sizes and the spatial
separation between the two mouths' components, so a near-miss and a
never-meant-to-connect read differently instead of both being "false".

No generation change, no threshold tuning. The fix is a design decision.

Build clean, 19 succeeded / 0 warnings / 0 failed. Sweeps 3.4 s + 6.9 s.

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

CODEMAP.md

@@ -304,7 +304,7 @@ This is **where terrain shape lives.**
 
 ### 3.6b Strate measurement — `Public/VoxelStrateMeasure.h` + `Private/VoxelStrateMeasure.cpp`
 Headless, read-only Tier 2 measurement pass. `VF_MeasureStrate` samples one strate between a
-boundary-seal-derived (or explicitly overridden) interior margin into a bounded coarse grid,
+boundary-seal-derived (or explicitly overridden, including explicit zero) interior margin into a bounded coarse grid,
 performs one deterministic 6-connected air flood fill, and derives fractions, components,
 walkability, feature scale, and clearance from that grid. Metrics report the resolved margin and
 the inclusive/exclusive voxel Z window used, plus the largest component's deterministic lowest-cell
@@ -318,9 +318,10 @@ because a corridor thinner than `SampleStep` is invisible. No UObject state, cac
 or PIE is required.
 | Symbol | Role |
 |--------|------|
-| `FVoxelStrateMeasureSettings` / `FVoxelStrateMetrics` | Plain settings/result structs for bounded strate sampling and derived measurements; callers may override the interior margin, and results identify the resolved margin/Z window and deterministic component representatives. |
+| `FVoxelStrateMeasureSettings` / `FVoxelStrateMetrics` | Plain settings/result structs for bounded strate sampling and derived measurements; callers may override the interior margin (including zero to sample the seal), and results identify the resolved margin/Z window and deterministic component representatives. |
 | `VF_MeasureStrate` | One-grid/one-flood-fill strate metrics; refuses invalid bounds or a grid over `MaxCells`. |
 | `VF_AreConnected` | Coarse 6-connected BFS plus one full-resolution recheck of its recovered route, with explicit endpoint-solid, out-of-window, `CoarseLied`, `NotConnectedAtThisResolution`, and connected outcomes. |
+| `VF_DiagnoseConnectivity` | The same query plus each mouth's air-component size/share and exact geometric distance to the nearest cell of the other component; proximity is measured in coarse-cell coordinates and does not claim a route through solid. |
 
 ### 3.7 Cave morphology (SDF rooms/tunnels) — `Public/VoxelCaveMorphology.h` + `.cpp`
 Header is rich with inline docs. Two namespaces + a per-chunk cache system.
@@ -485,6 +486,7 @@ The plugin's first tests (`OPSTACK-PLAN.md` Phase 0.5). Run them from the editor
 | `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 a VerticalShafts refinement sweep. The refinement test explicitly checks margins 0/2/4/8 at step 2/radius 192, then conditionally reports mouth components/proximity and a 16-seed arrival→departure result. |
 
 ## 4. The density pipeline (most-edited hot path)