test: stage A is bit-identical — strengthen the test its own counters indicted

The port is correct across 6000 samples. The only failure was arithmetic in
the assertion: 4 ops + 3 structural = 7, and I wrote 6.

But two numbers that PASSED are the real finding. 65 of 6000 samples landed
in open cave (1.1%), so the equivalence mostly compared solid rock to solid
rock — the carve, pits, chimneys and worm carve only run near the network.
And 3 of 400 probe points genuinely differed between the two param sets, so
the stale-cache check asked its question three times and said "fine" 397
times about points that could never have answered it.

Both counters were written to say exactly this, and did. Both guards only
fired at ZERO, so the run went green with the coverage of a much smaller
test. A coverage guard that only trips at zero does not measure coverage,
it notices absence. Both are now fraction thresholds that print a percent.

RoomSpacing 80 -> 42, RoomDensity 0.35 -> 0.85.

New check 3b: rebuild the stack with PitDensity = ChimneyDensity = 0 and
count the points that move. Enabling a feature in the params is not evidence
it fired — SDFCache.Pits can come back empty and the test stays green — and
these are the two loops DECOMPOSITION §2 calls the fiddliest in the plugin.

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

OPSTACK-PROGRESS.md

@@ -1992,3 +1992,57 @@ and the test's `GetGenerationParams` signature.
 `VoxelForge.OpStack.TunnelNetworkSpineEquivalence`. Then stage B (the 13 modifiers).
 
 ---
+
+## 2026-07-28 — Stage A is bit-identical. And its own counters say the test was thin.
+
+```
+TunnelNetwork STAGE A: bit-identical across 6000 samples in 24 chunks
+                       (65 in open cave, 5636 in rock)
+Params-fingerprint   : 3 of 400 probe points genuinely differ, 0 served wrong
+Box verdicts         : 0 proved, 40 Mixed          ← asserted, expected
+Op count             : expected 6, got 7           ← FAIL
+```
+
+**The port is correct.** Vertical scale, cave warp, the shared `BuildChunkCache`, pits and chimneys
+at unwarped coords, the floored carve divisor and the worm network mask all reproduce the original
+bit for bit. The only failure was **my arithmetic in the assertion**: 4 ops + 3 structural = 7, and I
+wrote 6. Fixed.
+
+### The two numbers that matter are the ones that passed
+
+**65 of 6000 samples in open cave — 1.1 %.** Green, and mostly comparing solid rock to solid rock:
+the carve, the pits, the chimneys and the worm carve only run near the network. At `RoomSpacing = 80`
+/ `RoomDensity = 0.35` a 32-voxel chunk usually contains no cave at all.
+
+**3 of 400 probe points differ between the two param sets — 0.75 %.** So the stale-cache check asked
+its question three times and answered "not stale" 397 times about points that could never have
+revealed staleness.
+
+Both counters exist precisely to say this, and both said it. **Both guards, however, only fired at
+ZERO** — so the run was green with the coverage of a much smaller test. That is the same lesson as
+"a test that prints nothing on success is indistinguishable from one that never ran", one notch
+finer: *a coverage guard that only trips at zero does not measure coverage, it only notices absence.*
+Both are now **fraction thresholds** (≥ 10 % cave, ≥ 5 % differing probes) and both print a
+percentage.
+
+Fixes applied: `RoomSpacing 80 → 42`, `RoomDensity 0.35 → 0.85`.
+
+### New check 3b — did the pits and chimneys actually fire?
+
+`§2` calls those two loops the fiddliest thing in the whole decomposition (unwarped coords
+`SmoothMin`'d into the warped room SDF). Setting `PitDensity = 0.55` proves nothing: `SDFCache.Pits`
+can still come back empty and the test stays green. So the new check builds the same stack with
+`PitDensity = ChimneyDensity = 0` and counts the points that MOVE. Zero ⇒ those loops contributed
+nothing and the equivalence says nothing about them, whatever colour the test is.
+
+Same move as the FloatingIslands "samples inside island rock" counter, applied to a sub-feature
+instead of a whole archetype: **enabling a feature is not evidence it fired.**
+
+**UNVERIFIED:** the strengthened test is not compiled. Only the test file changed; the operators are
+untouched and already proven bit-identical.
+
+**Next single action:** re-run the filter (13 tests). Expect stage A green with a cave fraction in
+the tens of percent, a fingerprint check in the double digits, and a non-zero pit/chimney count.
+Then stage B — the 13 detail modifiers.
+
+---