style: clippy-1.93 machine-applicable auto-fixes (#967)

cargo clippy --fix on files the new clippy (1.93) flags: unused imports
(name_index, storyteller), manual_range_contains (block_irregularity),
length-comparison/is_empty (layer3, serialization). All behavior-preserving.
Surfaced because a warm target/ makes the pre-push hook actually run clippy
(it skips on cold worktrees). Remaining non-auto-fixable test-code lints
tracked in #967.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/tests/layer3.rs

@@ -102,7 +102,7 @@ fn server_subprocess_sends_snapshot_on_connect() {
 
     // 8. Assert protocol correctness (D-020)
     assert!(
-        snapshot.entities.len() > 0,
+        !snapshot.entities.is_empty(),
         "snapshot should contain at least one entity (the player), got 0"
     );
 

server/tests/serialization.rs

@@ -1065,7 +1065,7 @@ fn gdscript_generated_fixtures_deserialize() {
     );
 
     let mut count = 0;
-    for entry in fs::read_dir(&fixture_dir).expect("read gdscript fixture dir") {
+    for entry in fs::read_dir(fixture_dir).expect("read gdscript fixture dir") {
         let entry = entry.expect("read dir entry");
         let path = entry.path();
         if path.extension().and_then(|e| e.to_str()) != Some("msgpack") {