Distinguish Guardian parent-compaction failure reasons

[copyberry]

Distinguish Guardian parent-compaction failure reasons

Why

The generic parent_compaction_error classification metric does not distinguish serialization failures, oversized compactions, and unusable compactions.

What changed

Record separate failure_reason values for each case: parent_compaction_serialization, parent_compaction_oversized, and parent_compaction_unusable.

Testing

Extend parent-compaction reuse tests to cover unusable checkpoints in parent-context mode and assert specific failure reasons. Verify rejected compactions receive the maximum risk score, provide no cached approval, and do not reuse stale context.

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