feat(composer): lateral REGIONS -- built, measured, and gated OFF

Jahni's own idea: "archetype could even mix within the same strate at some
point, tunnel network leading to a big chamber". An archetype becomes a
spatial REGION inside a strate, not a label on it.

Built by extending the mechanism that already exists rather than inventing
one: the stack blends VERTICALLY between strates, so regions blend the same
way LATERALLY. Deterministic jittered Voronoi sites on a 256-voxel lattice,
1-3 regions per strate from (seed, strateIndex), 24-voxel symmetric blend
band, per-chunk lookup cache. Parameters are never blended -- a shaft's
ShaftSpacing means nothing to a slab -- only the resulting DENSITY is.
Structural posts stay global and unconditional, so an invented region
cannot switch off the primordial law.

  single-region path   BIT-IDENTICAL to legacy, 256 probes, native + recipe
  cross-boundary boxes 20 of 40, 199,800 voxels brute-forced, 0 VIOLATIONS
                       (every seam-spanning box correctly returned Mixed)
  blend-band cost      663 ns vs 338 ns per voxel, over 16.0% of voxels
                       ~1.15x field-weighted
  survival             16/16 valid, non-vacuous, largest share >= .50

⛔ AND IT IS GATED OFF, because the seam breaks the one law that matters:
arrival -> departure across a region boundary passes 9/16 seeds, against
11/16 same-region controls in the same harness. Density blending alone does
not guarantee a player can cross a seam. A strate that mixes archetypes and
strands you at the boundary is strictly worse than one that does not mix.

VF_LateralRegionsAreShippable() returns false and the test ASSERTS THE GATE
IS SHUT while the rate is imperfect -- so flipping it without fixing
connectivity turns the suite red by design. The rate is a warning, not an
assertion: it is the measured state of an off-by-default feature, not a
regression in a live world, and a permanently red suite stops being read,
which is how real failures get missed.

Everything that ships is still hard-guarded: single-region bit-identity,
zero cross-boundary box violations, and every shipping-path law test
untouched.

The fix is an explicit CROSS-SEAM CORRIDOR CONTRACT -- a design decision,
recorded rather than made unilaterally. Codex diagnosed this correctly and
declined to tune the threshold, which was the right call.

Build clean, 29 succeeded / 0 failed, run and verified by me.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VJiTcTSDNkKBtnNdiBtdaU

CODEMAP.md

@@ -173,6 +173,22 @@ bit. They are port-correctness oracles, not fidelity checks: the acceptance bar
 | `VoxelDensityOps::BuildFloatingIslandStack` | — | 8 ops, and **the stack runs backwards**: void source + fill instead of rock source + carve, using the *same* classes with the opposite sign. Only the blob source is new. Reuse by **inversion** — a stronger result than reuse by identity, since it says the abstract axis (the density sign) is the right one. |
 | `VoxelDensityOps::BuildMazeStack` | — | The 8-op Maze stack (4 structural posts included). If this ever becomes one op, the refactor failed its own test (§2.5). Callers must skip it on a **degenerate strate** (top−bottom ≤ 0): `GetMazeDensity` early-outs to air there and the stack has no such early-out by design — `GetDensityAt` falls back to the `switch`. |
 
+#### Lateral region parent (Tier 4d)
+
+`VF_RollStrateRegionManifest` rolls a deterministic count of 1–3 regions and gives each region its
+own native parameter vector and, for structure candidates, its own recipe. `VF_QueryStrateRegion`
+assigns XY through jittered `VoxelHash::Cell` sites on a 256-voxel lattice; the pure query is keyed
+by `(WorldXY, Seed, StrateIndex)` and the worker-side `FVoxelStrateRegionPartitionCache` prepares
+the site window and integer query grid once per chunk. `FLateralRegionBlendOp` evaluates the primary
+region core and, inside a 24-voxel band, the nearest different-region core, then lerps **density**
+(not native parameters). The four structural posts are appended once to the parent after the
+blend. Count==1 uses the ordinary one-stack path and is covered by native and recipe bit-identity
+oracles. `ClassifyBox` proves a single-region/non-band box only from a conservative Lipschitz gap
+bound; otherwise it folds every region and returns `Mixed` on any disagreement or unknown. The
+runtime/editor override reads the immutable manifest per chunk; it never searches lattice seeds per
+voxel. The dedicated `VoxelForge.Composer.LateralRegions` test reports cross-boundary brute-force
+box samples, interior-vs-band timing, 16 opposite-region passage-mouth trials, and a saved preview.
+
 ### 3.2e Height-space operators — `Public/VoxelHeightOp.h` + `Private/VoxelHeightOpStack.cpp`
 ⚠️ **Feeds nothing yet** — built and exercised only by `VoxelForge.OpStack.SurfaceHeightEquivalence`.
 **A SECOND op family, and it exists for a reason worth knowing:** SurfaceWorld's terrain ops (cliff /