test: retain and localize channel GPU parity failures

tests/validation/multigrid/channel_parity_diagnostic/README.md

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+# Root-channel parity diagnostic
+
+This is a read-only diagnostic, not a replacement acceptance gate. It requires a
+caller-admitted GPU and the unchanged checkout containing
+`test_channel_root_gpu.py`. It does not run unrelated body/Jet tests.
+
+From the checkout, run:
+
+```
+python /path/to/channel_parity_diagnostic/discover.py --checkout "$PWD" --output /new/evidence/directory
+```
+
+Discovery invokes only the original `test_uniform_matched_root` method, with its
+original loops and exact assertion. It stops at the first assertion failure,
+extracts the configuration and arrays from that original traceback, saves the
+failure verbatim, and serially starts `replay.py` in a fresh Python process.
+The original failure remains a failure: exit 1 means the assertion was captured,
+even if the targeted replay succeeds or does not reproduce it. Exit 2 means the
+original test did not reproduce the failure; exit 3 means a different assertion
+failed and no configuration was guessed. Other errors are retained and raised.
+
+Targeted replay compiles only the uniform and one failed blocked configuration,
+serially, caching the two modules. It saves PTX and compiler metadata, all
+numeric and bit-only mismatches with coordinates/hex/ULPs at each step, first
+numeric divergence arrays, same-input localization, and comparison against the
+original multistep call. Signed-zero-only bit changes cannot trigger the first
+numeric divergence. The final replay consistency checks establish whether the
+one-step observation sequence agrees with the original call structure.
+
+Outputs retain the original source revision and gate SHA256. No source,
+assertion, tolerance, compiler flags, collision math, or kernel scheduling is
+changed. Findings must be reviewed before any production fix or gate change.