Debounce completion index rebuilds for action/command cache updates

[multiplex55]

Motivation

• Prevent expensive immediate rebuilds of the completion index on frequent watch events and avoid stale suggestions leaking during event bursts.
• Keep update_suggestions behavior unchanged while ensuring rebuilds are coalesced and only the relevant caches trigger a rebuild.

Description

• Add a short debounce window constant COMPLETION_REBUILD_DEBOUNCE and per-source dirty flags action_completion_dirty and command_completion_dirty on LauncherApp to track which part of the completion index needs rebuilding.
• Change update_action_cache and update_command_cache to mark only their respective dirty flag and call schedule_completion_rebuild instead of directly running update_completion_index.
• Introduce schedule_completion_rebuild, maybe_rebuild_completion_index, and rebuild_completion_index_now helpers to clear stale completion_index/suggestions, defer the rebuild for the debounce period, and handle immediate rebuild at startup.
• Call maybe_rebuild_completion_index at the end of process_watch_events so bursts of WatchEvents are coalesced; leave update_suggestions logic intact once the index is rebuilt.
• Add unit tests in the src/gui/mod.rs test module: watch_event_bursts_delay_completion_rebuild_until_debounce_window and completion_suggestions_clear_until_rebuild_and_match_latest_entries to validate debouncing and suggestion freshness.

Testing

• Ran cargo fmt which completed successfully.
• Attempted to run the new tests with cargo test, and specifically exercised watch_event_bursts_delay_completion_rebuild_until_debounce_window, but the test run could not complete in this environment because building the dependency graph failed due to a missing system library (alsa.pc) required by alsa-sys (system pkg-config dependency), so tests did not finish here.

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Codex Task