feat(opstack C3): TunnelNetwork + Underwater in the ported list -- 8 of 8

UsesOperatorStackForChunk now returns true for both, so the archetype switch has a complete
operator-stack twin: per-strate opt-in, every archetype equivalence-tested bit for bit against its
original function.

This changes nothing by itself. The flag still requires bUseOperatorStack ticked on a strate asset,
which is Jahni's call and was deliberately NOT done. What HAS changed is that the flag is no longer
a no-op anywhere: ticking it on any strate now really switches that strate onto the stack.

Not done, and it is the next real prize: ClassifyTile still uses hand-written guards and does not
consume ClassifyBox. That is where measured tile-skipping becomes frames.

DOCS
- CODEMAP 3.2d: ported list 6 of 8 -> 8 of 8; the BuildTunnelNetworkStack row rewritten (19 ops, one
  builder for two archetypes); six new rows for the detail modifiers, each carrying the thing a
  reader would otherwise have to rediscover -- roughness reads STRATE params, terrace re-queries the
  SDF, the cliff's comment disagrees with its code, columns have no strate parameter at all, and
  LocalParams() is the override whose EffectOverBox is too optimistic on a strate with an op pool.
  Also corrected the stale "never compare the two paths" line: C10 is closed and all eight
  equivalence tests compare bit for bit.
- CODEMAP 3.3 UsesOperatorStackForChunk row: same list, plus the warning that the flag is now a real
  switch rather than a harmless tick.
- OPSTACK-PLAN: status header and the Phase 2 order both updated; the three-stage TunnelNetwork
  breakdown and the calls-not-transcribes rule recorded there rather than only in the code.
- OPSTACK-PROGRESS: the closing entry for this unattended run -- every commit in order, the five
  original-code findings ported as-is, the two decisions that are not reversible by taste, the
  ClassifyBox optimism C1 introduced and that must be fixed before ClassifyTile consumes it, what
  breaks first per group, and the likely compile-error spots.

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

OPSTACK-PLAN.md

@@ -4,11 +4,18 @@
 > *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):** **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).
+> **Status (2026-07-28):** **Phases 0.5, 1 and 2 CODE-COMPLETE — 8 of 8 archetypes ported**, each
+> bit-identical to its original in an equivalence test, all wired behind `bUseOperatorStack`:
+> **Maze · FlatPlain · CrystalChamber · SurfaceWorld (biomes included) · VerticalShafts ·
+> FloatingIslands · TunnelNetwork · Underwater.** The archetype `switch` now has a complete
+> operator-stack twin, opt-in per strate.
+>
+> ⚠️ **The last three groups (stage B, C1, C2, C3) are UNVERIFIED — written, committed, never
+> compiled.** Run the `VoxelForge` filter; `OPSTACK-PROGRESS.md`'s last entry lists every commit and
+> what breaks first if one is wrong.
+>
+> **Not done, and it is the next real prize:** `ClassifyTile` still uses hand-written guards and does
+> not consume `ClassifyBox`. That is where measured tile-skipping becomes frames.
 >
 > 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
@@ -406,12 +413,27 @@ care: the T1.a column cache and the exact-lattice `ClassifyTile` bound both surv
 ✅ `VerticalShafts` (**done**, 3 ops reused from Maze unchanged) →
 ✅ `FloatingIslands` (**done**, `BuildFloatingIslandStack` — the stack that runs **backwards**: void
 source + fill instead of rock source + carve, the *same* classes with the opposite sign; only the
-blob source is new) → `Underwater` (TunnelNetwork + a flag) → `TunnelNetwork`
-(**last** — it owns `BuildChunkCache`'s two-region window-invariance discipline, §8.4, the most delicate
-code in the plugin).
+blob source is new) → ✅ `TunnelNetwork` + `Underwater` (**done**, one builder for both —
+`BuildTunnelNetworkStack`, 19 ops).
 
-**6 of 8 ported.** The two that remain are really one: `Underwater` *is* TunnelNetwork plus
-`WaterLevelRelative` (§8), so the switch loses its last two cases in a single port.
+**8 of 8 ported.** The last two were really one: `Underwater` *is* TunnelNetwork plus
+`WaterLevelRelative` (§8, re-verified before relying on it), so the switch lost its last two cases in
+a single port.
+
+TunnelNetwork was ~1080 lines and was taken in **three stages, each verifiable on its own** rather
+than as ~600 unverified lines on top of ~200 (the `AUDIT §P3` pattern):
+* **A** — SDF spine: vertical scale, base rock, cave warp, room graph (+ pits + chimneys), carve,
+  worms, structural post. Verifiable *while incomplete* because every detail modifier is
+  amplitude-gated and defaults to zero, so zeroing them sends the ORIGINAL down exactly stage A's path.
+* **B** — the twelve detail modifiers of `STEP 4b–4h`, one group per commit, each with a coverage
+  probe that proves the group actually moved something (`B1` roughness, `B2` terrace/lines/ribs,
+  `B3` overhang/cliff/scallop/arch, `B4` columns/domes/pinch/floor-bias, `B5` the gate itself).
+* **C** — the per-room op override (`§2`'s option (a), and it needed no scoping predicate: one op
+  owns the state, eleven read it), `Underwater`, and the flag flip.
+
+⚠️ **`FRoomGraphSource` CALLS `BuildChunkCache`/`EvaluateSDFCached`; it does not transcribe them.**
+That is where §8.4's two-region window-invariance discipline lives, and a copy would fork it — with
+the fork "validated" by a test that compares it to the original.
 
 Along the way, `FStrateGenerationParams`' 74 fields decompose into per-op structs, which retires the
 `VF_STRATE_PARAM_FIELDS` X-macro drift problem for free.