feat: the Mask combiner — biome blending in height space

Biome blending needs to ask which biome is at an XY, and the real answer is a warped
Voronoi with a per-chunk cache on UVoxelGenerator. The op must not hold a generator
pointer — Phase 3 wants ops to become assets, and one that owns a generator never
can. So it depends on IVoxelBiomeField, a two-line interface returning (dominant,
neighbour, weight), and the adapter that knows the generator stays on the generator's
side. Same move as cliff -> structural: depend on the capability, not the owner.

FBiomeBlendHeightSource holds one complete height stack per biome and lerps the
HEIGHTS in the border band. Each biome's stack computes its own relief and gates its
own terrace, exactly as the original makes two independent full calls and blends only
the outputs. Blending heights rather than params is what keeps borders continuous
across any param difference.

The ceiling SELECTS the dominant instead of blending, because that is what the
original does. Reproduced as-is rather than improved — a blended sky cap changes the
world's silhouette and a port is not where that gets decided.

Tested against a synthetic field rather than the real resolver: the resolver has its
own coverage, while a synthetic field sweeps the weight 0 -> 1 continuously, which is
where an inverted lerp hides. Five weights x 400 points, bit-exact against FMath::Lerp
of the two full stacks, plus a check that the ceiling still returns the dominant's at
weight 1.

UNVERIFIED: not compiled.

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

OPSTACK-PROGRESS.md

@@ -1530,3 +1530,58 @@ that could actually flip a sign. Recorded in `AUDIT §C9`.
 (the `Mask` combiner = biome blending, `§5`'s Phase 3 prototype), then `VerticalShafts` (`§6`).
 
 ---
+
+## 2026-07-27 — the `Mask` combiner (biome blending) in height space. §5's Phase 3 prototype.
+
+All 10 tests green on the previous build, and the reworded digest confirms the point that mattered:
+
+```
+NearIso profile over 115000 samples: 2 within 1e-4, 0 within 1e-5, 0 within 1e-6
+```
+
+**Zero in the tight band** — no sampled voxel sits close enough to the isosurface for a libm
+difference to flip its side. The residual `§C9` library half is real in principle and, on this grid,
+carries no measured risk. The wide band's "2" was the ~100× over-statement, exactly as predicted.
+
+### The design decision worth recording: `IVoxelBiomeField`
+
+Biome blending needs to ask "which biome is at this XY?", and the real answer is a warped Voronoi
+with a per-chunk cache living on `UVoxelGenerator`. **The op must not hold a `UVoxelGenerator*`** —
+Phase 3 wants operators to become *assets*, and an op that owns a generator pointer never can.
+
+So the op depends on `IVoxelBiomeField`, a two-line interface returning `(dominant, neighbour,
+weight)`. The adapter that knows the generator stays on the generator's side. This is the same move
+as `cliff → structural source`: depend on the *capability*, not on the owner.
+
+### The combiner itself
+
+`FBiomeBlendHeightSource` — **one complete height stack per biome**, heights lerped in the border
+band. Each biome's stack computes its **own** relief `M` and gates its **own** terrace with it,
+which is exactly what the original does (two independent full `ComputeSurfaceTerrainZ` calls, only
+the OUTPUTS blended). Blending *heights* rather than *params* is what keeps borders continuous
+across any param difference — interpolating params would drag a terrace through intermediate states
+that mean nothing.
+
+**The ceiling SELECTS instead of blending**, because the original takes the dominant biome's ceiling
+alone. Reproduced as-is rather than "improved": a blended sky cap would change the world's
+silhouette, and a port is not where that gets decided.
+
+### Tested against a synthetic field, on purpose
+
+The real Voronoi resolver has its own coverage; a synthetic field lets the weight sweep 0 → 1
+**continuously**, which is where an inverted lerp (`1-w` for `w`) hides — it looks right in the
+middle and wrong only at the ends. Two deliberately dissimilar biomes, five weights × 400 points,
+bit-exact against `FMath::Lerp` of the two full stacks. Plus a check that the ceiling still returns
+the dominant's at weight 1.0, which is the case a "blend everything" refactor would silently break.
+
+**UNVERIFIED:** not compiled. Likely spots: the local `class FFixedWeightField` declared inside a
+function body and used as an interface; `TArray<FVoxelHeightStack>` (move-only element type — needs
+`MoveTemp` on insert, which it has).
+
+**Next single action:** build. Then the density-side integration — `FSurfaceColumnSource` takes the
+per-biome params + field, blends the overhang amp (`Lerp(Amp(PD), Amp(PN), W)` with slope from PD),
+and the generator-side adapter wraps `ResolveBiomeSampleAt`. That drops the biome guard in
+`UsesOperatorStackForChunk` and finishes SurfaceWorld. Then `VerticalShafts` (§6), which reuses
+`FConstantRockSource` + `FSdfRoughnessMod` + `FSdfCarve` and should be the cheapest port yet.
+
+---