feat: index checksum verification

[Lucas-Bur]

What to build

Add checksum verification for index files (chunks.jsonl and vectors.bin) to detect corruption or malicious tampering. Store a SHA-256 checksum alongside the index at write time, verify at read time.

Acceptance criteria

• Index write stores SHA-256 checksum of both files (e.g., in .pix/checksums.json)
• Index read verifies checksums before loading
• Mismatch produces a clear error message (IndexCorruptionError)
• pix verify command to manually check index integrity
• --json output for verify command
• Adapter test verifies checksum mismatch detection
• Adapter test verifies valid checksum passes

Blocked by

None - can start immediately