Adopt rustfmt and clean up clippy warnings

Formats the whole backend to rustfmt defaults and fixes all 23
outstanding clippy warnings (collapsible ifs, clone-to-slice-ref,
type-complexity, single-arm match, unnecessary is_none check), then
wires cargo fmt --check and -D warnings into make lint so both stay
enforced going forward.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

src/engine/handle.rs

@@ -110,7 +110,11 @@ fn new_channel_and_handle(
     let reader = IndexReader {
         factory: read.factory(),
     };
-    (EngineHandle { tx, reader }, rx, IndexWriter { handle: write })
+    (
+        EngineHandle { tx, reader },
+        rx,
+        IndexWriter { handle: write },
+    )
 }
 
 pub fn spawn(settings: SettingsHandle, channel_capacity: usize) -> EngineHandle {