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_bridge/src/lib.rs

@@ -98,6 +98,12 @@ pub enum BridgeError {
     Unmapped(String),
 }
 
+impl BridgeError {
+    fn unmapped_ctx(ctx: impl std::fmt::Display, e: impl std::fmt::Display) -> Self {
+        BridgeError::Unmapped(format!("{ctx}: {e}"))
+    }
+}
+
 impl From<chanora_core::CoreError> for BridgeError {
     fn from(e: chanora_core::CoreError) -> Self {
         match e {