Phase 0 (miroir-qon): Foundation verification — formatting fix

Apply rustfmt to migration.rs for consistency with project style.

All Phase 0 DoD items verified:
- cargo build --all: ✅ passes
- cargo test --all: ✅ 42 tests pass
- cargo clippy: ✅ passes (without --all-features due to known openraft/Rust 1.87 incompatibility)
- cargo fmt --check: ✅ passes
- Config round-trip: ✅ tested
- Workspace, crates, config struct: ✅ complete

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/miroir-core/src/migration.rs

@@ -495,7 +495,11 @@ impl MigrationCoordinator {
         }
 
         // Clear only the in-flight writes for this migration
-        let affected_shards = state.affected_shards.keys().cloned().collect::<HashSet<_>>();
+        let affected_shards = state
+            .affected_shards
+            .keys()
+            .cloned()
+            .collect::<HashSet<_>>();
         self.in_flight
             .retain(|w| !affected_shards.contains(&w.shard));