Summary

• Add Fsck.check/2 with :full and :quick modes for repository integrity verification
• Add list_objects/2 callback to Storage behaviour, implemented in all three backends
• Full mode verifies: loose object SHA integrity, object graph completeness, ref validity, pack checksums
• Quick mode verifies: ref validity and graph reachability only

Closes #14

Changes

• lib/ex_git_objectstore/fsck.ex — New module with check/2 entry point
• lib/ex_git_objectstore/storage.ex — Added list_objects callback
• lib/ex_git_objectstore/storage/{memory,filesystem,s3}.ex — list_objects implementations
• test/support/tracking_memory.ex — Added list_objects delegation
• test/ex_git_objectstore/fsck_test.exs — 11 tests covering all acceptance criteria

Test plan

• Valid repo returns clean report
• Detects corrupted object data (SHA mismatch)
• Detects missing object referenced by tree
• Detects missing parent commit
• Detects dangling ref
• Detects corrupted pack checksum
• Detects missing tag target
• Quick mode detects dangling ref but skips SHA verification
• Report includes counts
• All 540 tests pass, dialyzer clean