feat: add multi-item ingest and Reed-Solomon parity

- Multi-item ingest: each item gets its own Unix socket, Rust processes
  them sequentially into a single snapshot with separate chunk lists
- Reed-Solomon parity: rs(20,1) erasure coding for pack file groups,
  enabling single-pack-loss recovery via parity reconstruction
- Repair now attempts parity-based recovery for missing pack files
- 16 integration tests + 12 Rust unit tests all pass

rust/src/main.rs

@@ -17,6 +17,7 @@ mod ingest;
 mod restore;
 mod verify;
 mod prune;
+mod parity;
 mod repair;
 
 #[derive(Parser, Debug)]