feat: initial implementation of content-addressed incremental backup engine

Rust-centric architecture with TypeScript facade following smartproxy/smartstorage pattern.
Core engine in Rust (FastCDC chunking, SHA-256, gzip, AES-256-GCM + Argon2id, binary pack files,
global index, snapshots, locking, verification, pruning, repair). TypeScript provides npm interface
via @push.rocks/smartrust RustBridge IPC with Unix socket streaming for ingest/restore.
All 14 integration tests pass.

rust/src/error.rs

@@ -0,0 +1,37 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum ArchiveError {
+    #[error("I/O error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("Configuration error: {0}")]
+    Config(String),
+
+    #[error("Data corruption: {0}")]
+    Corruption(String),
+
+    #[error("Encryption error: {0}")]
+    Encryption(String),
+
+    #[error("Not found: {0}")]
+    NotFound(String),
+
+    #[error("Repository is locked: {0}")]
+    Locked(String),
+
+    #[error("Invalid repository: {0}")]
+    InvalidRepo(String),
+
+    #[error("JSON error: {0}")]
+    Json(#[from] serde_json::Error),
+
+    #[error("{0}")]
+    Other(String),
+}
+
+impl ArchiveError {
+    pub fn to_error_string(&self) -> String {
+        format!("{}", self)
+    }
+}