feat(measure): one grid, one flood fill -- the strate measurement pass

Tier 2 slice 1 from COMPOSER-NOTES.md 3.4. A read-only, deterministic,
headless measurement of a built strate: sample a coarse grid once,
flood-fill the air once, derive every metric from that same grid.

What it answers today: air fraction, air component count and largest
component share (the swiss-cheese signal), walkable fraction, median
feature scale, median vertical clearance -- and whether two points are
actually connected.

The connectivity verdict is the point. Coarse sampling LIES about it in
the dangerous direction: a wall thinner than the sample step vanishes and
two sealed caverns read as joined. So VF_AreConnected recovers the actual
BFS route and re-walks that one corridor at one-voxel spacing; a true
verdict is never returned on coarse evidence alone. It fired: 2 of 21
routes were coarse false positives.

The test carries a negative control, because a green suite that cannot
fail is what hid the last defect. Two of them, in fact: a direct probe of
the known-solid inter-strate gap (AirFraction 0.00042), and -- Codex's
improvement on the spec -- a diff-layer box filled solid and measured
through VF_MeasureStrate itself, so an inverted sign is caught end to end
rather than around the function under test.

Build clean, 18 succeeded / 0 warnings / 0 failed.

KNOWN, fix in flight: the interior window excludes a whole chunk (32
voxels) at each end to avoid a 4-voxel seal band -- 8x too much. On the
fixture's 4-chunk strates that discards the floor, so the reported
FlatPlain/CrystalChamber air fractions of 0.999 describe the open band
above the floor, not the strate. The engine is right; the window is not.

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

CODEMAP.md

@@ -302,6 +302,19 @@ This is **where terrain shape lives.**
 > tile by the mesher drops tail octaves on coarse tiles (opt-in `LODOctaveDrop`, default 0 = off);
 > XY-field noise (heightfield/ceiling/relief/moisture) deliberately stays un-biased. §8.10.
 
+### 3.6b Strate measurement — `Public/VoxelStrateMeasure.h` + `Private/VoxelStrateMeasure.cpp`
+Headless, read-only Tier 2 measurement pass. `VF_MeasureStrate` samples one strate's strict
+interior 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.
+`VF_AreConnected` recovers a deterministic BFS parent path from the same kind of grid and checks
+only that route at full voxel resolution before returning true; a coarse false positive sets
+`bOutCoarseLied`. No UObject state, cache, actor, world, or PIE is required.
+| Symbol | Role |
+|--------|------|
+| `FVoxelStrateMeasureSettings` / `FVoxelStrateMetrics` | Plain settings/result structs for bounded strate sampling and derived measurements. |
+| `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. |
+
 ### 3.7 Cave morphology (SDF rooms/tunnels) — `Public/VoxelCaveMorphology.h` + `.cpp`
 Header is rich with inline docs. Two namespaces + a per-chunk cache system.