breadmill/breadsearch: narrow embedder lock scope, recover from poisoned mutexes, atomic index save + corrupt rebuild, fix GUI busy-poll, bump theme pin

- indexer.rs: embedder mutex is now locked only around each chunk's
  embed_document() call instead of the whole file's chunk loop, so a slow
  MIGraphX JIT compile on one file no longer blocks every query for minutes
- sync_ext.rs (new)/indexer.rs/main.rs/serve.rs: added MutexExt::lock_recover(),
  replacing every .lock().unwrap() so a panic on one thread (poisoning the
  mutex) logs a warning and recovers instead of cascading into every future
  lock().unwrap() call
- store.rs: save_index now writes to a .tmp sibling and renames atomically
  into place; Store::open recovers from an index that fails to load by
  wiping it and the SQLite files/chunks tables so the next scan rebuilds
  from scratch, instead of refusing to start at all. Note: this recovery
  only catches corruption usearch's loader reports as Err — verified
  experimentally that sufficiently garbled input segfaults the process
  instead, which no Rust-side handling can catch; the atomic save is what
  actually prevents the realistic mid-crash corruption case from arising
- breadsearch/src/main.rs: GUI query-result wait switched from
  glib::idle_add_local (re-invoked every main-loop tick, pegging a core for
  the whole wait) to a 15ms glib::timeout_add_local poll
- breadsearch/Cargo.toml: bread-theme pin bumped v0.2.8 -> v0.2.10 to match
  the rest of the family

Cargo.lock

@@ -123,7 +123,7 @@ dependencies = [
 [[package]]
 name = "bread-theme"
 version = "0.2.3"
-source = "git+https://github.com/Breadway/bread-ecosystem?tag=v0.2.8#5e58558dd36031433d4a8d8e70c71206c3f1f8f4"
+source = "git+https://github.com/Breadway/bread-ecosystem?tag=v0.2.10#17d1bb85801b9a8c195b64c02d288cd662c9c780"
 dependencies = [
  "dirs",
  "gtk4",