Observed-context auditor: repaired guard and verified frozen replay

Corrects the independent observed-context auditor's obsolete metadata-key guard and adds an actual pinned-protocol parser regression test.

Verified after the fix: 62 affected tests passed in 0.204 s; all 436 fast tests passed in 235.965 s with unchanged source hashes and no skips/exclusions. Corrected replay passed all 8,640 frozen episodes. Eight semantic/provenance corruption controls were rejected after a positive control.

The measured code-v97 runtime, protocol, raw data, manifest and numerical summaries remain unchanged. Verification uses its immutable source archive plus a separate current-execution hash check; only the auditor and its test differ, and their hashes are retained. The initial failure is preserved. No learning or performance improvement is claimed by this auditor fix. CI for this corrected source is run 39166387866; pending jobs are not credited as passing. Slow/retry and local full discovery were not rerun for the additive diagnostic path.