style: Apply cargo fmt and clippy fixes across codebase

Automated formatting and lint corrections from parallel agent work:

- cargo fmt: import reordering (alphabetical), line wrapping to respect
  max width, trailing comma normalization, destructuring alignment,
  function signature reformatting, match arm formatting
- clippy (pedantic): Range::contains() instead of manual comparisons,
  i64::from() instead of `as i64` casts, .clamp() instead of
  .max().min() chains, let-chain refactors (if-let with &&),
  #[allow(clippy::too_many_arguments)] and
  #[allow(clippy::field_reassign_with_default)] where warranted
- Removed trailing blank lines and extra whitespace

No behavioral changes. All existing tests pass unmodified.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/backoff.rs

@@ -86,7 +86,10 @@ mod tests {
         let result = compute_next_attempt_at(now, 1);
         let delay = result - now;
         // attempt 1: base = 2000ms, with jitter: 1800-2200ms
-        assert!(delay >= 1800 && delay <= 2200, "first retry delay: {delay}ms");
+        assert!(
+            (1800..=2200).contains(&delay),
+            "first retry delay: {delay}ms"
+        );
     }
 
     #[test]