feat: wire parity into ingest pipeline, optimize restore with nonce caching

- Parity generation auto-triggers after every N packs during ingest
- ParityConfig stored in repository config.json
- Nonce stored in global index entries, eliminating IDX re-reads during
  encrypted restore (fast path) with IDX cache fallback
- Repair now attempts parity-based pack reconstruction before reindexing

rust/src/config.rs

@@ -11,6 +11,16 @@ pub struct RepositoryConfig {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub encryption: Option<EncryptionConfig>,
     pub pack_target_size: u64,
+    #[serde(default)]
+    pub parity: Option<ParityConfigSpec>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ParityConfigSpec {
+    pub algorithm: String,
+    pub data_count: usize,
+    pub parity_count: usize,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -92,6 +102,11 @@ impl RepositoryConfig {
             compression: "gzip".to_string(),
             encryption,
             pack_target_size: 8 * 1024 * 1024, // 8 MB
+            parity: Some(ParityConfigSpec {
+                algorithm: "reed-solomon".to_string(),
+                data_count: 20,
+                parity_count: 1,
+            }),
         }
     }
 }