# x87 precision-sensitivity experiment — artefacts (lane F, 2026-09-08)

Read `findings/subsystems/fpu-precision-sensitivity.md` for the result and the argument.
This directory is the evidence.

Seven End-Turn runs from `C:\SOTS\SavedGames\ref-turn2.sav` (`ab4ac2d7…`) on VM140, shim build
`fpucw-cef889e-20260908T0803Z`. The hook set, trace path and flush policy are identical in every
run; the ONLY difference between the `shim.cfg` files is the `fpu.force=` line.

```
run-off/       nothing forced -- oracle re-confirmation + the game's own control-word timeline
run-027f/      fpu.force=0x027f   53-bit, nearest   (the MSVC CRT default; NOT 24-bit)
run-127f/      fpu.force=0x127f   53-bit, nearest   -- the CONTROL: what the game sets itself
run-137f/      fpu.force=0x137f   64-bit extended, nearest
run-007f/      fpu.force=0x007f   24-bit single, nearest
run-1a7f/      fpu.force=0x1a7f   53-bit, round-toward-+infinity
run-007f-rep/  repeat of run-007f -- reproducibility of the divergence
run-1a7f-rep/  repeat of run-1a7f
```

Each run directory holds `(Autosave).sav` (post-turn, the thing under test),
`(Autosave EndTurn).sav` (pre-turn, the run-to-run control -- `bb4fd9ac…` in every run),
`shim.log` (the force / read-back / per-tick-sampler timeline), `shim.cfg` and the gzipped
trace. The two `-rep` runs keep only the saves and the log.

Derived, regenerate with the commands below:

* `cw-census.txt` — the `fpu_cw` every hooked call observed, per run. This is the evidence that
  the forced word held for the *duration* of the turn: 38 samples per run spanning turn phases
  4, 6 and 8, all reading the forced value.
* `state-checksum-diffs.txt` — every run's post-turn autosave against `run-127f`.
* `trace-bitdiff.txt` — bit-exact trace comparison. Note the trap it exists for: under a forced
  24-bit control word the CRT's own `%g` rendering degrades, so trace *text* is not a valid
  comparison surface.

```sh
uv run python3 verify/fpu-cw/cw_census.py verify/results/fpu-cw/run-*/shim.trace.jsonl.gz
uv run python3 verify/state-checksum/state_checksum.py \
    "verify/results/fpu-cw/run-127f/(Autosave).sav" "verify/results/fpu-cw/run-007f/(Autosave).sav"
uv run python3 verify/fpu-cw/trace_bitdiff.py \
    verify/results/fpu-cw/run-127f/shim.trace.jsonl.gz verify/results/fpu-cw/run-1a7f/shim.trace.jsonl.gz
```

Headline hashes (sha256 of the file, gzip container included -- MTIME is 0, so this is valid):

```
bb4fd9ac89f41e3bc0db2af08b18ce83417521ac4bcee695fc9fa6ce16e30948  (Autosave EndTurn).sav  ALL SEVEN RUNS
978041acd168b56ed8eb3f5e42e78d5e70eae6e6517d75e659a5eb7ca3d60921  (Autosave).sav  stock / 0x027f / 0x127f / 0x137f
ba2435beb17265af3abddb58cd26e926d42b39b358ef6d0c76f9897386174918  (Autosave).sav  0x007f  (both runs)
e48e25fa8425a8a1d174efe3e8a553153447f11f8f91c5fda25bd7db3a26cca7  (Autosave).sav  0x1a7f  (both runs)
```

The first two lines are also the **determinism-oracle re-confirmation** on engine `cef889e`
(lane M's movement fix included): unchanged, as it must be — that fix touches `ours`, never the
original.