test: fix the ULP yardstick — it measured output density, not where the error is born

The tuned pass warned: 121/20000 differ, 6 over the bound, worst 1.72e-05, still 0
isosurface crossings. The port is fine; the bound was wrong.

It was 16 * max(|Old|, 1) * FLT_EPSILON — ULPs on the OUTPUT density. But density is
min(Z - Floor, Ceil - Z), so near the isosurface the output tends to 0 while the
intermediates are in the hundreds. Rounding born at scale ~400 judged against a
yardstick of scale 1: 400x too tight, and tightest exactly where the test looks
hardest. Large amplitudes are what expose it, which is why the tuned pass earned
its place immediately.

Measured rather than assumed: amplitudes rose x2.25-3.33 and the deltas rose x4.5,
with the worst delta at 0.345 ULP of |Z| — sub-ULP at the scale it is born in. Error
proportional to amplitude is ordinary rounding. A wrong noise offset or a missing
abs() would move the surface by voxels, four orders of magnitude above this.

The bound now scales with max(|Old|, |Z|, strate Z bounds), and the warning prints
the discriminator instead of just the alarm: the density at the offending sample and
the delta in ULPs of the working scale. A few ULP at near-zero density is
cancellation; thousands is drift. That distinction is now readable rather than
re-derivable at a build apiece.

The box verdicts held under the worst case: 32/60 proved uniform, 0 unsound, under
tripled ceiling roughness and 3x the columns — exactly the case that stresses the
Max(CeilZ - noise, FloorSurface + 2) clamp.

Also recorded in DECOMPOSITION section 3: FlatPlain and CrystalChamber render
identical in the live world because nothing in the content distinguishes them. The
merge loses no distinction; it reveals there was none.

UNVERIFIED: the corrected bound.

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

OPSTACK-PROGRESS.md

@@ -990,3 +990,72 @@ question is whether it still reads as the same kind of place. Then `SurfaceWorld
 (OPSTACK-DECOMPOSITION §5) — biggest payoff, most care.
 
 ---
+
+## 2026-07-27 — the tuned pass cried wolf. The TEST was wrong, not the port.
+
+```
+CrystalChamber(tuned): 121/20000 differ, 6 exceed the bound, worst |delta| 1.71661377e-05
+                       0 cross the isosurface;  box verdicts 32/60 proved, 0 unsound
+```
+
+**The port is fine. The yardstick was wrong, and it was wrong in a way that only shows up under
+large amplitudes — which is precisely why the tuned pass was worth adding.**
+
+The old bound was `16 · max(|Old|, 1) · FLT_EPSILON` — ULPs measured on the **output density**.
+But the density is `min(Z - Floor, Ceil - Z)`, so **near the isosurface the output tends to 0 while
+the intermediates (surfaces, world Z, noise amplitudes) are in the HUNDREDS.** A rounding born at
+scale ~400 was being judged against a yardstick of scale 1: 400× too tight, and tightest exactly
+where the test looks hardest.
+
+**Measured, not assumed:**
+
+| | roughness | worst \|delta\| | in ULP of \|Z\| |
+|---|---|---|---|
+| gentle passes | ceil 6, floor 4 | 3.81e-06 | ~0.08 |
+| tuned pass | ceil 20, floor 9 | 1.72e-05 | **0.345** |
+
+Amplitudes went up ×2.25–3.33, the deltas went up **×4.5**, and the worst one is **sub-ULP at the
+scale it is born in**. Error proportional to amplitude is the signature of ordinary rounding, not of
+a wrong transcription — a wrong offset or a missing `abs()` would move the surface by **voxels**,
+four orders of magnitude above this, not by a factor of four.
+
+**Fixed:** the bound now scales with the magnitude the error is born in (`max(|Old|, |Z|, strate Z
+bounds)`), and — more importantly — **the warning now prints the discriminator instead of just the
+alarm**: the density at the offending sample and the delta expressed in ULPs of the working scale.
+A few ULP with a near-zero density is cancellation; thousands of ULP is drift. The next context
+reads that off the message instead of re-deriving it at a build apiece.
+
+The test stays discriminating: real drift is 4 orders of magnitude above the new bound.
+
+### The box verdicts held under the worst case — that is the result that mattered
+
+32 of 60 proved uniform with **0 unsound**, under tripled ceiling roughness and 3× the columns.
+That was the specific thing the tuned pass existed to attack (a wide ceiling band makes the
+`Max(CeilZ - noise, FloorSurface + 2)` clamp bind, which is where a false verdict would be a HOLE),
+and the bound survived it. Fewer tiles proved than the gentle passes (32 vs 36/40), which is correct
+— wider bands mean more genuinely Mixed tiles.
+
+### Visual A/B — and a content finding worth more than the A/B
+
+Jahni: *"visually, crystal and plain are identical, same for when opting on or off from the
+opstack."*
+
+**Opt-in on/off identical = the port is confirmed a pure refactor.** Note this is the expected
+result and my earlier framing was sloppy: **both** paths compute the post-§3.1 function, so the
+Z-term change is invisible in the A/B by construction. It is only visible against the world as it
+looked *before* this build.
+
+**FlatPlain and CrystalChamber rendering identical is the real finding.** They share
+`FSlabGenerationParams` and nothing in the content distinguishes them — **the enum promised a
+difference the data never delivered**, in the shipped world exactly as in the test fixture. So the
+merge loses no distinction; it reveals there was none. Recorded in `OPSTACK-DECOMPOSITION §3`.
+Making a crystal chamber look like one is now a **params** job (`CeilingRoughness` 6 → ~20), which
+is precisely the outcome the refactor is for.
+
+**UNVERIFIED:** the corrected bound and the new warning text.
+
+**Next single action:** rebuild; the tuned pass should drop to INFO with all three passes reporting
+sub-ULP-of-scale deltas. Then `SurfaceWorld` (§5) — biggest payoff, most care: the T1.a column cache
+and the exact-lattice `ClassifyTile` bound must both survive the port.
+
+---