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_cache/src/lib.rs

@@ -29,6 +29,12 @@ pub enum BlobCacheError {
     InvalidKey(String),
 }
 
+impl BlobCacheError {
+    fn io_ctx(ctx: impl std::fmt::Display, e: impl std::fmt::Display) -> Self {
+        BlobCacheError::Io(format!("{ctx}: {e}"))
+    }
+}
+
 /// Content-addressed blob cache backed by cacache.
 pub struct BlobCache {
     cache_dir: PathBuf,
@@ -51,7 +57,7 @@ impl BlobCache {
         // cacache creates the directory on first write, but we create
         // it eagerly so total_size() works before any writes.
         std::fs::create_dir_all(&cache_dir)
-            .map_err(|e| BlobCacheError::Io(format!("mkdir cache: {e}")))?;
+            .map_err(|e| BlobCacheError::io_ctx("mkdir cache", e))?;
         Ok(Self {
             cache_dir,
             max_bytes,
@@ -73,7 +79,7 @@ impl BlobCache {
         let cache_key = format!("{prefix}{key}");
         cacache::write(&self.cache_dir, &cache_key, data)
             .await
-            .map_err(|e| BlobCacheError::Io(format!("cacache write: {e}")))?;
+            .map_err(|e| BlobCacheError::io_ctx("cacache write", e))?;
         Ok(())
     }
 
@@ -106,7 +112,7 @@ impl BlobCache {
         let cache_key = format!("{prefix}{key}");
         cacache::remove(&self.cache_dir, &cache_key)
             .await
-            .map_err(|e| BlobCacheError::Io(format!("cacache remove: {e}")))?;
+            .map_err(|e| BlobCacheError::io_ctx("cacache remove", e))?;
         Ok(())
     }
 
@@ -116,14 +122,14 @@ impl BlobCache {
         tokio::task::spawn_blocking(move || {
             if path.exists() {
                 std::fs::remove_dir_all(&path)
-                    .map_err(|e| BlobCacheError::Io(format!("clear cache: {e}")))?;
+                    .map_err(|e| BlobCacheError::io_ctx("clear cache", e))?;
                 std::fs::create_dir_all(&path)
-                    .map_err(|e| BlobCacheError::Io(format!("recreate cache dir: {e}")))?;
+                    .map_err(|e| BlobCacheError::io_ctx("recreate cache dir", e))?;
             }
             Ok(())
         })
         .await
-        .map_err(|e| BlobCacheError::Io(format!("clear task: {e}")))?
+        .map_err(|e| BlobCacheError::io_ctx("clear task", e))?
     }
 
     /// Return total bytes used by all blobs.
@@ -148,7 +154,7 @@ impl BlobCache {
             Ok(total)
         })
         .await
-        .map_err(|e| BlobCacheError::Io(format!("total_size task: {e}")))?
+        .map_err(|e| BlobCacheError::io_ctx("total_size task", e))?
     }
 
     /// Evict oldest entries by timestamp until total size is under
@@ -198,7 +204,7 @@ impl BlobCache {
             Ok(())
         })
         .await
-        .map_err(|e| BlobCacheError::Io(format!("evict task: {e}")))?
+        .map_err(|e| BlobCacheError::io_ctx("evict task", e))?
     }
 }