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/src/simulation/name_index.rs

@@ -184,7 +184,7 @@ fn collect_names(conn: &Connection) -> rusqlite::Result<Vec<(String, String)>> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use aho_corasick::{AhoCorasick, AhoCorasickBuilder, MatchKind};
+    use aho_corasick::{AhoCorasickBuilder, MatchKind};
 
     /// Build a minimal index directly (no DB) for unit testing.
     fn make_index(pairs: &[(&str, &str)]) -> SystemNameIndex {