fix: resolve all 40 clippy warnings (dead_code, unused_imports, lints)

- Add #[allow(unused_imports)] on pub re-exports used only by lib/tests
- Add #[allow(dead_code)] on public API items unused in binary target
- Fix collapsible_if, redundant_closure, unnecessary_map_or in main.rs
- Prefix unused test variables with underscore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/audio/cache.rs

@@ -103,6 +103,7 @@ impl VoiceCache {
     }
 
     /// Clears all cached files
+    #[allow(dead_code)]
     pub fn clear(&mut self) -> Result<(), String> {
         for (path, _, _) in self.files.values() {
             let _ = fs::remove_file(path); // Ignore errors