feat(opstack): tunnels get a threshold test -- T1.d is measured at 6 of 40 tiles, 0 violations

THE PRIZE LANDED. At production defaults: 6 of 40 tiles proved AllSolid, 7986
voxels brute-forced, 0 violations. 15% of tiles skip GenerateMesh entirely, one
BuildChunkCache traded against 30000+ density evaluations each, and not one
verdict is asserted -- every voxel was re-evaluated and came back on the claimed
side. The dense fixture still reports 0, exactly as the arithmetic said it must,
so this is a measurement of production rather than of a widened test.

The breakdown finally isolated the tunnels: of 34 unproved tiles, 32 were
blocked by a tunnel and 21 by a room, so >=13 were tunnel-only. That is what
justified the deferred disjunction, and why it was deferred rather than guessed.

A primitive no longer matters if it misses its cull OR its own SDF stays >= T+K
over the box. Each branch wins on a different class, by calculation: for rooms
the cull (Rmax+3K) is tighter than the threshold (Rmax+T+K), so rooms keep the
cull alone; for tunnels the cull is a capsule BOUNDING SPHERE, radius ~107 for a
200-long tube of radius 7, against a true segment distance. Order of magnitude.

The bound is exact, not cautious. TaperedCapsule was read, not assumed --
Dist(P, ClosestOnSegment) - Lerp(Ra,Rb,t) -- so SDF >= dist(P,segment) -
max(Ra,Rb); and dist(box,segment) >= dist(centre,segment) - half-diagonal by
triangle inequality. VF_DistPointSegment is written locally rather than taken
from FMath: five lines, and "I think that function does that" is not good enough
under a correctness bound.

Identity CHANGED MEANING, from "Sdf stays FLT_MAX" to "Sdf >= T" with
T = max(3*SDFBlendRadius, WormNetworkRange). Sound only because all three
consumers of the SDF channel were read one by one: FSdfConvertOp's Blend is
MakeSdfCarve(P.SDFBlendRadius, ...) => K; the twelve modifiers gate at 3K;
FWormFieldSource at WormNetworkRange. Plus the one that could have bitten --
FCaveTerraceMod re-probes the SDF at Z+-1, OUTSIDE the box, but its gate is line
13 and the probes are lines 28-29, so a gate false everywhere never emits one.
ANY NEW CONSUMER OF THE Sdf CHANNEL MUST HAVE A THRESHOLD <= T OR JOIN THAT MAX,
or it gets tiles with no geometry and no collision. Written at the site.

Why -K suffices for any N: SmoothMin's penalty is exactly zero once |A-B| >= K,
so the running minimum saturates at K below the smallest term and cannot descend
further. Sdf >= min_i(SDF_i) - K for ANY number of primitives, not - N*K/6 --
without which the slack would scale with the ~88 tunnels and be worthless.

Unbuilt.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CODEMAP.md

@@ -154,7 +154,7 @@ bit. They are port-correctness oracles, not fidelity checks: the acceptance bar
 | `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 the per-column memo, keyed by `PrepareChunk` on `(StrateBottomWorldZ, LayoutVersion, Seed)` so it is **shared down the whole vertical strate stack**, exactly like `GSurfColCache`. |
 | `VoxelDensityOps::BuildSurfaceStack` | — | SurfaceWorld, complete: column + overhang + 3 structural, plus biome blending when `PerBiomeParams` is non-empty. Takes ownership of an `IVoxelBiomeField`. |
 | `VoxelDensityOps::BuildVerticalShaftStack` | — | 8 ops, and **three are Maze's reused unchanged** (`ConstantRock`, `SdfRoughness`, `SdfCarve`) with different tuning (freq 0.1 vs 0.12, window `rough+4` vs `R+rough+2`). The measured proof of `OPSTACK-PLAN §2.5`'s reuse claim. |
-| `FRoomGraphSource` (internal) | 1 | TunnelNetwork's SDF spine. **CALLS `BuildChunkCache`/`EvaluateSDFCached` — does not transcribe them**: that is where §8.4's two-region discipline lives and a copy would fork it. Owns the cave warp (scope = this op alone; pits/chimneys read *unwarped* coords, which is why no FRAME op was needed). Its cache key adds a **params CRC + LayoutVersion**; the original lacked them until AUDIT §C2 was fixed (2026-07-28) and now carries them too. **`EffectOverBox` ANSWERS SPATIALLY** since 2026-07-28: it builds the cache for the queried box into a *second* per-worker cache (never `FState::Cache`), then lifts each primitive's per-voxel cull from point to box — rooms/tunnels as spheres vs the warp-dilated box, pits/chimneys vs the **undilated** box (real coords), columns as infinite cylinders. `Identity` ⇒ `Sdf` stays `FLT_MAX` ⇒ `FSdfConvertOp` **and all twelve detail modifiers** go identity with it. Verdict is memoised per box (the twelve all ask the same question). Warp dilation uses a **provable** `\|Perlin3D\| ≤ 2`, not the header's observed `~[-1,1]`. |
+| `FRoomGraphSource` (internal) | 1 | TunnelNetwork's SDF spine. **CALLS `BuildChunkCache`/`EvaluateSDFCached` — does not transcribe them**: that is where §8.4's two-region discipline lives and a copy would fork it. Owns the cave warp (scope = this op alone; pits/chimneys read *unwarped* coords, which is why no FRAME op was needed). Its cache key adds a **params CRC + LayoutVersion**; the original lacked them until AUDIT §C2 was fixed (2026-07-28) and now carries them too. **`EffectOverBox` ANSWERS SPATIALLY** since 2026-07-28 — this is the T1.d switch (measured: **6 of 40 tiles proved AllSolid at production defaults, 7986 voxels brute-forced, 0 violations**). It builds the cache for the queried box into a *second* per-worker cache (never `FState::Cache`), then applies a **disjunction** per primitive: it doesn't matter if it **fails its cull** *or* if **its own SDF stays ≥ `T+K`** over the box. Cull wins for rooms (`Rmax+3K` < `Rmax+T+K`); the threshold wins hugely for tunnels, whose cull is a capsule *bounding sphere* (~107 radius for a 200-long tube of radius 7). ⚠️ **`Identity` therefore means `Sdf ≥ T`, not `Sdf == FLT_MAX`**, with `T = max(3·SDFBlendRadius, WormNetworkRange)` — **any new consumer of the `Sdf` channel must have a threshold ≤ T or be added to that max**, or it gets tiles with no geometry and no collision. The `−K` slack covers any number of primitives because `SmoothMin`'s penalty is exactly 0 once `\|A−B\| ≥ K`. Pits/chimneys use the cull only; columns are **not** tested (their sole consumer gates on `Sdf`, so the test was redundant). Verdict memoised per box; warp dilation uses a **provable** `\|Perlin3D\| ≤ 2`. |
 | `FWormFieldSource` (internal) | 1 | Fielded 3D-noise threshold carve, masked by distance to the room network (reads `InOut.Sdf` *after* pits/chimneys). **`EffectOverBox` INHERITS `FRoomGraphSource`'s verdict** since 2026-07-28 — its `Eval` sets `NetworkMask = 0` when `CaveSDF >= WormNetworkRange`, which `FLT_MAX` always satisfies, so where the room source proves `Identity` the worm doesn't execute at all. ⚠️ This was **the** blocker: `BaseDensity = 8` < `WormStrength = 10` **by default** (the field comment requires it), so an unconditional `CarveOnly` drove `SolidMargin` negative on every tile in the world and no room-source proof could survive behind it. Deliberately **not** `VF_NoCaveOverBox` — that helper answers "identity" for a null `Rooms`, which is wrong for an op that could sit behind a different SDF writer. |
 | `VoxelDensityOps::BuildTunnelNetworkStack` | — | **COMPLETE, 19 ops** — the biggest port in the plugin (~1080 lines), done in three stages: SDF spine (A) → the twelve detail modifiers of 4b–4h (B) → the per-room op override (C). Serves **TunnelNetwork and Underwater** from one builder. Operator order is the original's, line for line, and it is load-bearing (`FFloorBiasMod` exists to undo what `FCaveRoughnessMod` did to floors). |
 | `FCaveRoughnessMod` (internal) | 3 | STEP 4b, **density space** — a different op from `MakeSdfRoughnessMod`: two octave sets, optional domain warp, four noise types, an anti-fill clamp inside definite air, quadratic fade. ⚠️ **Reads STRATE params, not the per-room copy** — the original's shadow is declared *after* step 4b. Eleven of twelve modifiers read the room copy; this one does not. |