Fix window close / quit safety races

[georgeharker]

Fixes several independent race conditions and correctness issues in the window close / app quit path. All changes are in VimR/VimR/ with no dependencies on other in-flight work.
AppDelegate.swift — applicationShouldTerminate

Both cancel paths (blocked-windows alert dismiss and dirty-windows cancel) returned without calling reply(toApplicationShouldTerminate:). Since the function returns .terminateLater, this left the app permanently stuck in a pending-termination state — subsequent window closes or Cmd+Q would misbehave because macOS considered termination already in progress. Added reply(false) before each early return.

MainWindow+Actions.swift / MainWindow+Delegates.swift / MainWindow.swift — dead closeWindow flag
The closeWindow IBAction set a closeWindow = true flag before calling performClose, causing windowShouldClose to send :qa! (quit all tabs) instead of :q (close current tab). Cmd+W should only close the current tab. Removed the flag, the defer reset, and the entire :qa! branch. windowShouldClose now always takes the closeCurrentTab / :q path.
MainWindow+Delegates.swift — ipcBecameInvalid

Called windowController.close() directly, bypassing neoVimStopped() / prepareClosing(). This meant isClosing was never set, the CLI pipe was not closed, and the Redux .close action was never emitted — leaving the window's UUID permanently in app state. Routed through neoVimStopped() instead.
UiRoot.swift — subscriber early-return race

The guard if self.mainWindows.isEmpty { return } was intended for app startup but also fired when a rapid second state update arrived after the last window had already been removed — silently preventing afterLastWindowAction (quit/hide) from ever firing. Restructured to only trigger quit/hide when windows were actually removed in this update and the result is empty (guard !uuidsToRemove.isEmpty, self.mainWindows.isEmpty).