docs: park the ULP residue as AUDIT C10; strip the diagnostic scaffolding

Jahni's call to pin it and move on, and the right one -- six builds spent and the
information stopped being worth the cost.

The final run closed it as far as it can be: SDFs identical everywhere (counted
unconditionally, 0 differ), yet two character-identical carve implementations in
the SAME translation unit fed a provably identical input differ by 1 ULP on
126/5000. That is only possible if they compile to different instruction
sequences, which /fp:fast permits based on surrounding context with no single
isolable axis. Hypothesis 3 was right about the mechanism and wrong about every
clean variable proposed for it, which is why four well-designed isolation tests
came back negative.

AUDIT C10 records the observation, what is proven (SDF bit-exact 126/126, zero
isosurface crossings), the five refuted hypotheses in a table so nobody repeats
them at a build each, why the settling experiment is blocked (shared-PCH / IWYU
debt), and the rule that actually matters: never run both density paths in one
world and never compare them for equality. That is NOT a client-desync risk --
within a binary the field is proven bit-pure and every peer runs the same path --
the cross-platform concern is C9, which stands on its own.

Corrected OPSTACK-PLAN 2.6 and C9: my earlier "/fp:fast across translation units"
explanation was measurably wrong and is removed rather than softened.

MazeEquivalence keeps the permanent value (equivalence with ULP grading,
window-invariance, box-verdict brute force) and drops the verbatim copy,
three-way, bisect, inlining and constness experiments.

Phase 1 closed: Maze decomposes into 7 ops, SDF bit-exact, 0 isosurface
crossings, window-invariant, and 23 of 60 tiles proved uniform where ClassifyTile
proves zero.

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

OPSTACK-PROGRESS.md

@@ -727,3 +727,49 @@ both cases correct `OPSTACK-PLAN §2.6` / `AUDIT §C9` / the test's INFO text, s
 scaffolding, and resume step 3.
 
 ---
+
+## 2026-07-27 — ULP residue PARKED by decision. Phase 1 closed. Moving to step 3.
+
+**Jahni's call, and the right one:** pin it and move on. Six builds spent; the information stopped
+being worth the cost.
+
+**The final run did close it as far as it can be closed:**
+
+```
+my carve(verbatim's own SDF) == verbatim : 4874 / 5000
+stack SDF  !=  verbatim SDF              : 0      (counted directly, no condition)
+```
+
+SDFs identical **everywhere**, not just among mismatches. Two character-identical carve
+implementations, in the **same translation unit**, fed a **provably identical** input, differ by
+1 ULP on 126/5000. For deterministic code that is only possible if they compile to different
+instruction sequences — which is exactly what `/fp:fast` permits based on surrounding context, with
+no single isolable axis. **Hypothesis 3 was right about the mechanism and wrong about every clean
+variable I proposed for it**, which is why four carefully-designed isolation tests all came back
+negative.
+
+The one experiment that would settle it (`FPSemantics = Precise` on this module) is blocked behind
+the shared-PCH / IWYU debt. Worth clearing on its own terms; not worth clearing to chase 1 ULP.
+
+**Recorded as `AUDIT-2026-07.md §C10`** with the full refutation table, so the next context cannot
+re-derive the same five hypotheses at a build each. `OPSTACK-PLAN §2.6` and `§C9` corrected — my
+earlier `/fp:fast`-across-TUs explanation was wrong and is gone.
+
+**Scaffolding stripped** from `MazeEquivalence`: the verbatim copy, the three-way, the bisect, the
+inlining/constness experiments and the worst-point dump are all out. What remains is the permanent
+value: the equivalence check with ULP grading, the window-invariance check, and the box-verdict
+brute force.
+
+### Phase 1 is closed. What it proved
+
+- Maze decomposes into **seven** ops with no contortion; three are already shared with other archetypes.
+- The **SDF is bit-exact** — lattice, hashes, `Capsule` all correct.
+- **Zero isosurface crossings** — geometrically identical to the original.
+- **Window-invariant** across query order and worker threads.
+- **Every box verdict survives brute force**, and **23 of 60 tiles prove uniform** where
+  `ClassifyTile` proves zero for any cave archetype. That is the perf case, measured.
+
+**Next single action:** Phase 1 step 3 — wire the stack into `GetDensityAt` behind a per-strate
+opt-in, so a Maze strate can be A/B-switched in the editor and judged on a screenshot (§2.6's bar).
+
+---