feat: TunnelNetwork stage A — the SDF spine, wrapping BuildChunkCache

The last archetype is ~1080 lines with 13 detail modifiers, a two-region
cache and a per-room op override. Porting it whole before anything can be
verified is ~600 unverified lines on top of ~200 — the pattern this
refactor has dodged six times. So: three stages.

Stage A = vertical scale, base rock, cave warp, room graph (+ pits and
chimneys), carve, worms, structural post. 6 ops. It is verifiable NOW
because every detail modifier is amplitude-gated and FStrateGenerationParams
already defaults them all to zero — zeroing SurfaceRoughness sends the
ORIGINAL down exactly the path stage A ported.

TunnelNetwork stays OFF in UsesOperatorStackForChunk until stage C.

The decision that matters: FRoomGraphSource CALLS BuildChunkCache and
EvaluateSDFCached rather than transcribing them. That is where §8.4's
two-region window-invariance discipline lives; a transcription would fork
it, and the fork would be "validated" by a test comparing it to the
original. Only the ~60 lines of glue are transcribed.

FRAME ops are retired. All three candidates are now ported and none needed
one: CaveWarp's scope is exactly one operator (pits/chimneys read unwarped
coords), VerticalScale is a one-line pure function, and the island warp was
already local. Not missing infrastructure — one idea seen three times from
a distance.

Also: check 3 was going to compare two interleaved param sets against the
original, which would have FAILED — the original's SDF cache key has no
params, so it serves B the rooms it built for A. Comparing there measures
its bug, not the port. Rewritten against each stack evaluated alone. The
same reasoning suggests a live production staleness across Gradient
transitions; filed in AUDIT §C2 as SUSPECTED with the check that would
confirm it, since it rests on a premise I have not verified.

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

OPSTACK-DECOMPOSITION.md

@@ -170,6 +170,22 @@ The order is load-bearing and is the order the code already uses: spine carves t
 the seal then re-solidifies its bands (the spine deliberately never touches them), passages punch
 through everything including the seal, and the player wins last.
 
+> ### ⛔ RETIRED 2026-07-28 — frames were never a fifth thing. Porting all three candidates killed it.
+>
+> The section below argues for a `FRAME` op family from three examples. All three are now ported,
+> and none of them turned out to need one:
+>
+> - **`CaveWarp` wraps exactly ONE operator.** Pits and chimneys explicitly read *unwarped* coords
+>   while writing the same SDF channel — the thing this document called "the single fiddliest thing
+>   in the whole decomposition". Inside one operator the difficulty evaporates: the warp is a local
+>   variable, not an inherited context. A transform whose scope is one op is not a frame.
+> - **`VerticalScale` is `Z / Scale`** — a pure function of a scalar and a param, recomputed in one
+>   line by each op that needs it. A frame would add a channel to avoid a division.
+> - **The island warp (§7)** was kept local for the same reason, before the other two were even read.
+>
+> **Zero frames out of three candidates.** It was not missing infrastructure; it was one idea seen
+> three times from a distance. Kept below as the reasoning that was superseded, not as a plan.
+
 ### A fifth thing the plan doesn't name: FRAME OPS
 
 Two archetypes transform the *query coordinates* rather than the field: