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-PROGRESS.md

@@ -1676,3 +1676,50 @@ cache's direct indexing vs. my hash, then per-voxel virtual calls.
 the flag off. Then `VerticalShafts` (§6).
 
 ---
+
+## 2026-07-27 — VerticalShafts ported. 5 of 8, and operator reuse is now MEASURED.
+
+**The port that tests the thesis rather than the fidelity.** Every previous port asked "does the
+decomposition reproduce the original?". This one asks **"do operators actually get reused across
+archetypes?"** — which is `OPSTACK-PLAN §2.5`'s claim and the only reason to do this refactor rather
+than tidy the `switch`.
+
+**Three of the five ops are Maze's, reused without a line changed:** `ConstantRock`, `SdfRoughness`,
+`SdfCarve`. 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** — frequency 0.1 instead of 0.12, window `rough + 4` instead
+of `R + rough + 2`. If the test is bit-identical, reuse stops being an intention and becomes a
+measurement.
+
+**Two new ops:** `FShaftFieldSource` (infinite cylinders + hash-gated connectors → SDF) and
+`FShaftLedgeMod` (banded shelves on the +X/+Y half only, so the shaft stays climbable).
+
+**Deviation from `§6`, stated:** it suggested splitting the source in two (XY-pure cylinders +
+connectors) so the cylinder half could get an exact XY box verdict. Kept as one op, because the
+connectors derive from the *same* 3×3 roll as the shafts and the ledge mod needs the shaft list
+anyway — splitting would mean rolling twice or sharing a cache between two ops. What is forfeited is
+the exact verdict on the cylinder half alone; what is kept is a conservative `EffectOverBox` that
+tests circles *and* connector reach. Revisit if the profile says it matters.
+
+**One subtlety worth flagging:** `FShaftLedgeMod` gates on `InOut.Sdf < 0` — the SDF **after**
+roughness, as the pile left it. Re-deriving the SDF there would give the pre-roughness value and
+shift every ledge. The op reads the channel rather than recomputing, which is exactly what the
+two-channel `FVoxelOpSample` is for.
+
+**Compile fix on the way in:** `FCells` was declared at the bottom of the class but returned by
+functions above it. Member *bodies* are deferred; *return types* are not — C4430 plus an unreadable
+cascade from a trivial cause. Moved up beside `FShaft`/`FConn`, with a note.
+
+**Ported: Maze · FlatPlain · CrystalChamber · SurfaceWorld (biomes incl.) · VerticalShafts — 5 of 8.**
+Remaining: `FloatingIslands` (§7), `Underwater` (§8, TunnelNetwork + a flag), and `TunnelNetwork`
+(§2) **last**, because it owns `BuildChunkCache`'s two-region window-invariance discipline (§8.4),
+the most delicate code in the plugin.
+
+**UNVERIFIED:** the shaft port is not compiled past the `FCells` fix.
+
+**Next single action:** build, run the `VoxelForge` filter — 11 tests now, the new one is
+`VoxelForge.OpStack.VerticalShaftEquivalence`. Then `FloatingIslands`. **Perf is deliberately parked
+until the transition is complete** (Jahni's call); the open item is that the op path is slower, with
+virtual dispatch and the hashed column lookup as the remaining suspects.
+
+---