Classify verify-raw diagnostics on whole phrases, after stripping colour codes

[MaxGhenis]

Follow-up to #334, fixing the diagnostic nit from its review.

The problem

chronicle verify-raw decides between missing_remote and fetch_error by looking for marker text in Wrangler's stderr. The review of #334 found two misclassifications. Both only affect entries that already fail, so neither could ever report verified:

• Colour codes inside the missing-object phrase made it a fetch_error.
• Unrelated text that merely mentioned NoSuchKey made it missing_remote.

The fix

_is_missing_object_diagnostic in chronicle/artifacts.py now:

• strips ANSI colour sequences and collapses whitespace before matching;
• matches a marker only as a whole phrase, so NoSuchKeyHandler isn't NoSuchKey;
• counts a marker only on a line that holds nothing but markers (such as NoSuchKey: specified key does not exist) or is an error line (it contains error, failed or Wrangler's ✘).

Nothing else in verify-raw changes.

Invariants and how they're checked

• Colour codes anywhere in the message still give missing_remote. One test inserts a colour sequence at every position of [ERROR] The specified key does not exist.
• A marker inside a longer identifier, or on a line that reports no error, gives fetch_error. Five cases cover this.
• Existing behaviour holds. The bare markers, the S3-style Code: message line and Wrangler's real captured stderr all still give missing_remote.

Tests

• tests/test_chronicle_verify_raw.py: 36 passed (25 existing, 11 new).
• tests/test_chronicle_artifacts.py: 36 passed.
• Three in-memory mutants each fail the new tests: the old substring logic (8 failures), no colour stripping (2) and never-missing (7).
• Live, with the real Wrangler and this branch's code: an object absent at its canonical key reports missing_remote, and the published IRD wage and salary workbook reports verified.
• Ruff 0.14.10 check and format pass.

Repository tooling only: no source package, fact or bundle pin changes.

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