feat: Phase 1 step 3 — wire the operator stack into GetDensityAt behind an opt-in

One branch on the density path, as OPSTACK-PLAN section 4 specified, and both
systems coexist.

- UVoxelStrateDefinition::bUseOperatorStack: the A/B switch section 2.6's
  acceptance bar needs. Flip it, regenerate, judge on a screenshot.
- UVoxelStrateManager::UsesOperatorStackForChunk(): the ported-archetype list,
  written down in exactly one place. An unported archetype ignores the flag and
  falls back to the switch, so ticking the box anywhere is harmless today and
  only Maze changes behaviour.
- GetDensityAt: CP_OpStack / CP_UseOpStack are resolved inside the SAME refetch
  block as the params, so the existing chunk + LayoutVersion key already covers
  them and there is no new invalidation logic to get wrong.

Hot-path cost is one bool test per voxel; the stack is built per chunk, the same
cadence as the param refetch. ApplyDisturbances and the diff layer stay outside
the stack and run once for both paths, so the tail of the pipeline is unchanged.

If UsesOperatorStackForChunk ever returns true for an archetype with no builder,
the code clears the flag and falls back to the switch rather than generating an
empty stack. An unported world is recoverable; a wrong one is not.

UNVERIFIED: not compiled. Likely spots: the `else switch` form, FVoxelOpStack as
a thread_local (move-only, reset by move-assigning a temporary), and the new
include.

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

OPSTACK-PROGRESS.md

@@ -773,3 +773,41 @@ brute force.
 opt-in, so a Maze strate can be A/B-switched in the editor and judged on a screenshot (§2.6's bar).
 
 ---
+
+## 2026-07-27 — Phase 1 step 3: the stack is WIRED IN, behind a per-strate opt-in.
+
+**`OPSTACK-PLAN §4` Phase 1 step 3 done.** `GetDensityAt` gains exactly one branch, as the plan
+specified, and both systems now coexist.
+
+**Files:**
+- `VoxelStrateDefinition.h` — `bool bUseOperatorStack` (EditAnywhere, "Use Operator Stack
+  (experimental)"). The A/B switch §2.6's acceptance bar needs: flip it, regenerate, judge the
+  screenshot.
+- `VoxelStrateManager.{h,cpp}` — `UsesOperatorStackForChunk()`. **The ported-archetype list lives
+  here and nowhere else**, so an unported archetype ignores the flag and falls back to the switch.
+  Ticking the box on any strate is therefore harmless today; only `Maze` changes behaviour.
+- `VoxelGenerator.cpp` — `CP_OpStack` / `CP_UseOpStack` built in the SAME refetch block as the
+  params (so the chunk+`LayoutVersion` key already covers it, no new invalidation logic), plus one
+  `if (CP_UseOpStack)` on the dispatch.
+
+**Cost on the hot path: one bool test per voxel.** The stack is built per chunk, never per voxel —
+the same cadence as the existing param refetch. `ApplyDisturbances` and the diff layer are
+deliberately left OUTSIDE the stack and run once for both paths, exactly as before, so the tail of
+the pipeline is untouched.
+
+**Defensive choice worth noting:** if `UsesOperatorStackForChunk` ever returns true for an archetype
+with no builder, the code clears the flag and falls back to the switch rather than generating an
+empty stack. A world that is *unported* is recoverable; a world that is *wrong* is not.
+
+**UNVERIFIED:** not compiled. Likely error spots: the `else switch` form in `GetDensityAt`,
+`FVoxelOpStack` as a `thread_local` (it is move-only — move-assign from a temporary is used to
+reset it), and the new include in `VoxelGenerator.cpp`.
+
+**What to look at in the editor:** set a Maze strate's `bUseOperatorStack`, regenerate, and compare
+against the same seed with it off. **Pass = recognisably the same maze** — same corridor scale, same
+connectivity, same feel. That is §2.6's bar, and it is the last thing Phase 1 needs.
+
+**Next single action:** build, then the visual A/B. After that, Phase 2 — the port order in
+`OPSTACK-DECOMPOSITION §10.4` starts with FlatPlain + CrystalChamber collapsing into one op.
+
+---