test: C9 is only half fixed — the libm half survives FPSemantics

All 9 tests green. LargeSeedSurvives proves C1 fixed by property rather than by
comparison: seed 2e9 (what FMath::Rand produces) now yields 400 distinct heights
over 400 samples where it previously gave a constant field. The three equivalence
tests stayed green through an 85-site rewrite.

The digest's NearIso warning fired at 2/115000 and its text blamed the /fp:fast vs
precise split, which is fixed. First instinct was "stale warning, soften it". Checked
instead, and the risk is real by a different mechanism:

sinf/cosf are not specified by IEEE-754. FPSemantics = Precise makes MSVC and Clang
agree on expression evaluation and says nothing about the math library; MSVC's CRT
and glibc's libm may differ by ~1 ULP. FMath::Sin/Cos are used throughout the density
path — layer lines, ribs, room placement, rotations. So C9's compiler half is closed
by construction and its library half is not, and no build flag can close it.

The measurement was also over-stating by ~100x: a single 1e-4 band is far too wide
for a libm-scale delta (~1e-6 absolute at densities of magnitude ~10). Replaced with
a three-band profile; only the tight band warns.

UNVERIFIED: the reworded test.

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

OPSTACK-PROGRESS.md

@@ -1479,3 +1479,54 @@ The three equivalence tests should stay green (both paths changed together); the
 op stack: the `Mask` combiner (biome blending) to finish SurfaceWorld, or `VerticalShafts` next.
 
 ---
+
+## 2026-07-27 — all 9 tests green. C1 PROVEN fixed. And C9 is only HALF fixed.
+
+Full `VoxelForge` filter, exported run `Saved/Automation/Automation2026.07.27-16.35.18.csv`:
+
+```
+ClassifyTileSoundness · CrossPlatformDigest · DensityPurity · DiffLayerContention
+LargeSeedSurvives · LiveEditInvalidation · BoxVerdictFold · MazeEquivalence
+SlabEquivalence · SurfaceHeightEquivalence          — all Success
+```
+
+**`§C1` is proven fixed, by property rather than by comparison:**
+
+```
+Seed 1337        : 400 distinct heights / 400 samples, range 20.95 voxels
+Seed 100 000     : 399 / 400, range 23.46
+Seed 10 000 000  : 399 / 400, range 22.89
+Seed 2 000 000 000 : 400 / 400, range 23.42
+```
+
+Seed 2e9 — what `FMath::Rand()` produces — now generates a live world. Before the fix that seed
+gave a *constant* field. Note the three equivalence tests stayed green through an 85-site rewrite,
+which is exactly the port-correctness value they were kept for.
+
+### ⚠️ The digest warning was right for the WRONG reason — and I nearly dismissed it
+
+`NearIso: 2 of 115000 samples within 1e-4 of the isosurface` fired a warning whose text blamed the
+`/fp:fast`-vs-precise split. That split is fixed, so my first instinct was "stale warning, soften the
+text". **I checked instead of assuming, and the risk is real via a different mechanism.**
+
+**`sinf`/`cosf` are not specified by IEEE-754.** `FPSemantics = Precise` makes MSVC and Clang agree on
+*expression evaluation*; it says nothing about the math library. MSVC's CRT and glibc's libm may
+legitimately differ by ~1 ULP — and `FMath::Sin`/`Cos` are all over the density path: layer lines
+(`VoxelGenerator.cpp:2275`, `VoxelHeightOpStack.cpp:232`), ribs, room placement, rotations.
+
+**So `§C9` is half closed:** the compiler half by construction, the **library half still open**. No
+build flag can fix the second — two libm implementations cannot be made to agree by a compiler
+setting. The fix, if it is ever needed, is a deterministic in-house `sin`/`cos` in the density path
+(one more world re-tune).
+
+**The measurement was also over-stating things ~100×.** A single 1e-4 band is far too wide for a
+libm-scale delta (~1e-6 absolute on densities of magnitude ~10). Replaced with a three-band profile
+(1e-4 / 1e-5 / 1e-6); only the tight band raises a warning, because only it corresponds to a delta
+that could actually flip a sign. Recorded in `AUDIT §C9`.
+
+**UNVERIFIED:** the reworded digest test.
+
+**Next single action:** build (quick — one test file changed). Then, per Jahni: finish SurfaceWorld
+(the `Mask` combiner = biome blending, `§5`'s Phase 3 prototype), then `VerticalShafts` (`§6`).
+
+---