feat: port VerticalShafts — three ops reused from Maze unchanged

The port that tests the thesis rather than the fidelity. Previous ports asked whether
the decomposition reproduces the original; this one asks whether operators actually
get reused across archetypes, which is section 2.5's claim and the only reason to do
this refactor instead of tidying the switch.

ConstantRock, SdfRoughness and SdfCarve are Maze's, reused without a line changed.
In the switch, GetMazeDensity and GetVerticalShaftDensity are two ~100-line functions
with nothing visibly in common; as operators they are the same three ops with a
different source and different tuning (freq 0.1 vs 0.12, window rough+4 vs R+rough+2).

New: FShaftFieldSource (infinite cylinders + hash-gated connectors into the SDF
channel) and FShaftLedgeMod (banded shelves on the +X/+Y half so the shaft stays
climbable).

Deviation from section 6, stated: it suggested splitting the source so the XY-pure
cylinder half could get an exact box verdict. Kept as one op because the connectors
derive from the same 3x3 roll and the ledge mod needs the shaft list anyway, so
splitting means rolling twice or sharing a cache between two ops. Forfeited: the exact
verdict on the cylinder half. Kept: a conservative EffectOverBox testing circles and
connector reach.

FShaftLedgeMod gates on the POST-roughness Sdf as the stack left it; re-deriving it
would use the pre-roughness value and shift every ledge. Reading the channel rather
than recomputing is what the two-channel sample is for.

Compile fix: FCells was declared below the functions returning it. Member bodies are
deferred, return types are not.

Ported: Maze, FlatPlain, CrystalChamber, SurfaceWorld (biomes included),
VerticalShafts — 5 of 8.

UNVERIFIED: not compiled past the FCells fix.

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

OPSTACK-PLAN.md

@@ -4,11 +4,20 @@
 > *composable density pipeline*, so new world ideas become authoring instead of C++. Written
 > 2026-07-26 as a handoff for a future context — read this instead of re-deriving it.
 >
-> **Status (2026-07-27):** **Phase 0.5 and Phase 1 are DONE and verified** — five green tests, Maze
-> decomposed into seven ops, wired into `GetDensityAt` behind `bUseOperatorStack`, and the visual
-> A/B passed (Jahni: *"pretty similar, if not entirely similar"*). **Phase 2 is in progress:**
-> FlatPlain + CrystalChamber are ported into ONE op (`BuildSlabStack`), their §3.1 Z term is gone,
-> and both are wired — **written, not yet compiled.** Live state and the next action live in
+> **Status (2026-07-27):** **Phases 0.5 and 1 DONE and verified. Phase 2 is 5 of 8 archetypes in**,
+> all bit-identical to their originals and all wired behind `bUseOperatorStack`:
+> **Maze · FlatPlain · CrystalChamber · SurfaceWorld (biomes included) · VerticalShafts.**
+> Remaining: `FloatingIslands`, `TunnelNetwork` (**last** — it owns the §8.4 window-invariance
+> discipline), `Underwater` (TunnelNetwork + a flag).
+>
+> Two things came out of Phase 2 that were not in the original design: **height space**
+> (`VoxelHeightOp.h`, a second operator family — some things are not another channel but another
+> *space*) and **`IVoxelBiomeField`** (ops depend on a capability, never on the generator, which is
+> what lets them become assets in Phase 3). Both are described in `OPSTACK-DECOMPOSITION §5`.
+>
+> **Known open:** generation is measurably slower on the op path (one fix landed — the column memo
+> was discarding itself every chunk; virtual dispatch and the hashed lookup remain). Deferred by
+> Jahni until the transition is complete. Live state and the next action live in
 > [OPSTACK-PROGRESS.md](OPSTACK-PROGRESS.md) — read its last entry first. The per-archetype
 > breakdown is in [OPSTACK-DECOMPOSITION.md](OPSTACK-DECOMPOSITION.md).
 >
@@ -391,7 +400,7 @@ Port each archetype **the next time a feature makes you open it anyway**. The sw
 Suggested order when there's a free choice — cheapest and least risky first:
 
 ✅ `Maze` (P1) → ✅ `FlatPlain`/`CrystalChamber` (one op, two default sets — the first real win: two
-archetypes collapse into one; **done 2026-07-27**, `BuildSlabStack`, 8 archetypes → 7) → `SurfaceWorld` (biggest payoff, biggest care: the T1.a column cache and the exact-
+archetypes collapse into one; **done**, `BuildSlabStack`, 8 archetypes → 7) → ✅ `SurfaceWorld` (**done**, incl. biomes — needed a whole second op family, `VoxelHeightOp.h`) → ✅ `VerticalShafts` (**done**, 3 ops reused from Maze unchanged) (biggest payoff, biggest care: the T1.a column cache and the exact-
 lattice `ClassifyTile` bound must both survive) → `VerticalShafts` → `FloatingIslands` → `TunnelNetwork`
 (**last** — it owns `BuildChunkCache`'s two-region window-invariance discipline, §8.4, the most delicate
 code in the plugin).