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/Cargo.toml

@@ -33,6 +33,9 @@ hex = "0.4"
 rand = "0.8"
 byteorder = "1"
 
+# Reed-Solomon erasure coding
+reed-solomon-erasure = "6"
+
 # Error handling
 thiserror = "2"
 anyhow = "1"