Effects: the sim golden test accepts macOS 27's libm hash for Mr. Mercury (#232)

On macOS 27.2 (rustc 1.98) one of the 404 pinned simulation hashes,
CC Mr. Mercury at non-default settings, 32 bpc, t = 2.2 s, came out
different. The reporter rendered every case at the commit that pinned
the hashes and at current main on that machine, and the frames were
byte-identical. So the code didn't change: the platform's libm did
(the hashes are exact f32 bits through sin, cos and powf). The test
now also accepts listed per-case libm variants, with that hash as the
first, and every other case stays exact. Its docs say how to tell a
libm difference from a code change.

Test: libm_variants_are_accepted_for_their_case_only (sim_golden; runs
on every platform).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/testing.md

@@ -19,7 +19,8 @@ cargo xtask ci             # fmt, clippy -D warnings, tests, layers, assets, was
   which read the clock are declared time-dependent. `crates/effects/tests/sim_golden.rs` pins
   exact pixel hashes of the simulation effects the GPU shares plans with (CC Rainfall … Card
   Wipe) at several settings, times, bit depths and resolutions, so any change to their CPU output
-  fails (re-pin with `SIM_GOLDEN_PRINT=1` after an intended change; pinned for aarch64 macOS).
+  fails (re-pin with `SIM_GOLDEN_PRINT=1` after an intended change; pinned for aarch64 macOS,
+  with the hashes another macOS release's libm gives for unchanged code in `LIBM_VARIANTS`).
 - **Export** (`crates/export/tests`): every format is encoded and decoded back, checking frame
   count, size and pixels. When `ffmpeg`/`ffprobe` are installed they are used as an outside
   check; they are never linked or shipped.