fix: guard against nil chunk in bucket.Get to prevent panic on corrupted cache load

[cubic-dev-ai]

Summary

• Adds a nil check for the chunk in bucket.Get before any slice expressions are evaluated, preventing a runtime panic when corrupted persisted cache data points to an unallocated chunk slot
• When a nil chunk is encountered, the corruption counter is incremented and the entry is treated as a cache miss (consistent with existing corruption handling)
• Adds a test verifying that Get on a nil chunk returns a miss and increments the corruption counter instead of panicking

Context

When cache data is loaded from a corrupted file via LoadFromFile*, map entries (b.m) may reference chunk indices that fall within len(chunks) but point to nil slots (chunks beyond the saved chunksLen are never allocated). The existing guard checked chunkIdx >= uint64(len(chunks)) but did not verify the chunk itself was non-nil, allowing chunk[idx : idx+4] to panic.

Test plan

• New test TestBucketGetNilChunkNoCorruptionPanic verifies nil chunk produces a miss with corruption counter incremented
• All existing tests pass (go test -count=1 ./...)
• go vet ./... clean

🤖 Generated with Claude Code

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Summary by cubic

Prevent panic in bucket.Get when corrupted cache data points to a nil chunk. Nil chunks now return a miss and increment the corruption counter instead of crashing.

• Bug Fixes
  • Added nil check for chunk before any slice ops in bucket.Get.
  • Treat nil chunk as miss and increment corruptions.
  • Added test TestBucketGetNilChunkNoCorruptionPanic to verify behavior.

^{Written for commit 97e7b90. Summary will update on new commits.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.81%. Comparing base (cef9ae9) to head (97e7b90).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   76.68%   76.81%   +0.12%     
==========================================
  Files           4        4              
  Lines         549      552       +3     
==========================================
+ Hits          421      424       +3     
  Misses         73       73              
  Partials       55       55              

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[cubic-dev-ai]

No issues found across 2 files