refactor: reduce map_err boilerplate with context helper methods (TODO-025)

Add per-crate variant_ctx() helper methods to StorageError, BlobCacheError,
ProtocolError, and BridgeError. Replace 78 .map_err(|e| format!(...))
closures with concise context calls. Identical error messages preserved.

crates/chanora_protocol/src/lib.rs

@@ -120,3 +120,13 @@ pub enum ProtocolError {
     #[error("file transfer failed: {0}")]
     FileTransfer(String),
 }
+
+impl ProtocolError {
+    fn lost(msg: impl Into<String>) -> Self {
+        ProtocolError::Lost(msg.into())
+    }
+
+    fn backend_ctx(ctx: impl std::fmt::Display, e: impl std::fmt::Display) -> Self {
+        ProtocolError::Backend(format!("{ctx}: {e}"))
+    }
+}