feat: replace SHA-256 with xxHash3-64 for file hashing

[cebarks]

Summary

• Replaces sha2 crate with xxhash-rust (xxh3 feature) for all file hashing — ~15x faster across all file sizes
• Swaps compute_file_hash, compute_hash, and HashingWriter in src/spt/mods.rs from SHA-256 to xxHash3-64
• Adds DB migration (022) to clear stale SHA-256 hashes so they get lazily recomputed
• Hash output changes from 64-char hex to 16-char hex; all consumers treat hashes as opaque strings

Convoy catalog generation, integrity checks, mod install/update, and backup/restore all benefit. Benchmark results (release mode, 20 iterations avg):

Size	SHA-256	xxHash3-64	Speedup
1 KB	528 ns	59 ns	8.9x
64 KB	32 µs	1.65 µs	19.5x
1 MB	424 µs	24.9 µs	17.0x
10 MB	4.27 ms	264 µs	16.1x
50 MB	21.5 ms	1.40 ms	15.3x

Test plan

• All 825+ existing tests pass
• Verified mod install produces 16-char xxHash3-64 hashes in DB
• Verified integrity check detects tampered files using new hash algorithm
• Migration clears old hashes; existing code handles NULL hashes gracefully

Implemented with the help of Claude Code

[smashedbanan]

sick benchmark results