Fix corrupted local cache eviction with logging and tests

[cchristous]

Summary

• Evict corrupted local cache entries instead of just logging warnings. Previously, corrupted metadata (e.g., missing outputID from stale entries across builds) caused repeated warn-level log spam on every cache check. Now corrupted entries are cleaned up and treated as cache misses.
• Add debug-level logging when evicting corrupted entries for observability without spam. Warn-level logging is used for actual eviction failures (permission denied, I/O errors).
• Guard against empty outputID in S3 Get responses to prevent corrupted entries from propagating into the local cache. Returns a clean cache miss consistent with other metadata parse failures.
• Add comprehensive tests covering corrupted metadata eviction, valid entry preservation, empty outputID defense-in-depth, permission error resilience, and read-only mode GET-after-PUT behavior.

Why these changes are safe

• The eviction in check() only fires for non-ErrNotExist errors from readMetadata — genuine cache misses (file not found) are unaffected.
• The evict() function tolerates already-removed files (ErrNotExist is silently ignored) and logs real filesystem errors at Warn level.
• The S3 empty outputID guard follows the existing pattern for all other metadata parse failures in S3.Get() — returns miss=true, err=nil and properly closes the response body.
• Debug logging for eviction avoids production log spam while remaining visible with -debug for troubleshooting.
• 8 new tests validate both positive and negative cases including permission error resilience.

Test plan

• TestCorruptedMetadata_EvictsEntry — corrupted metadata triggers eviction of both data and meta files
• TestCorruptedMetadata_ValidEntryNotEvicted — valid entries are not affected by eviction logic
• TestEmptyOutputID_BackendHit_LocalCacheSelfHeals — empty outputID from backend creates corrupted entry that self-heals on next check
• TestEvict_PermissionError_LogsWarningAndContinues — eviction failure due to permissions does not panic, files remain
• TestReadOnlyMode_GetAfterPut — GET after PUT in read-only mode hits local cache
• All existing tests pass