Cache todo links index by todo/note version signals

[multiplex55]

Motivation

• Reduce repeated work rebuilding the notes+links index on every todo links query by caching the computed (notes, index) when source data hasn't changed.
• Ensure the cached index is invalidated when todos or notes change so query results stay correct.
• Provide a way to observe rebuilds from tests to verify caching behavior.

Description

• Added a small cache type TodoLinksIndexCache and a process-wide TODO_LINKS_INDEX_CACHE keyed by (todo_version(), note_version()) to store notes and the LinkIndex in src/plugins/todo.rs.
• Introduced get_todo_links_index() which returns a cached (notes, index) when versions match and otherwise rebuilds the index and updates the cache while incrementing TODO_LINKS_INDEX_REBUILD_COUNT.
• Wired cache invalidation into existing todo update paths by calling invalidate_todo_links_index_cache() from update_cache() and the file-watcher closure so watcher-triggered updates also clear the cache.
• Replaced the inline index rebuild in the todo links query path with a call to get_todo_links_index() and added a unit test todo_links_reuses_cached_index_between_queries that asserts rebuild count behavior.

Testing

• Ran cargo fmt -- src/plugins/todo.rs which completed successfully.
• Added unit test todo_links_reuses_cached_index_between_queries exercising cache reuse and invalidation behavior and confirmed it is present in the test suite.
• Attempted to run the targeted test (todo_links_reuses_cached_index_between_queries), but cargo test failed in this environment due to a missing system dependency required by alsa-sys (missing alsa.pc), so the test could not be executed here.

---

Codex Task