fix(ui): Prevent keymap leakage and harden persist/config reliability

[mhiro2]

Summary

• Prevent popup keymaps from leaking into source buffers when using buffer_mode = "source"
• Sanitize invalid close_events config entries with graceful fallback to defaults
• Always invoke on_done callback in persist operations even when disabled
• Use synchronous save on VimLeavePre to ensure session data is persisted before Neovim exits

Changes

• 4f2dbe9 : fix(ui): prevent source mode keymap leakage
  • Skip installing popup-specific keymaps when buffer_mode = "source" to avoid polluting real editing
    buffers
  • Add test to verify keymaps are never added to source buffers
• 7bc6d3c : fix(config): sanitize close_events with fallback
  • Filter out non-string and empty entries from close_events arrays instead of rejecting the entire config
  • Fall back to default events when the sanitized list is empty or the value is not a table
• a5e6562 : fix(persist): always resolve on_done when disabled
  • Call on_done(false) in save_current and restore when persist is disabled
  • Ensures callers can rely on the callback being invoked in all code paths
• aeee02f : fix(persist): use sync save path on leave
  • Switch to synchronous file write during VimLeavePre to guarantee data is saved before exit
  • Add write_sync function to store module with atomic write pattern (tmp file + rename)