Closed windows never release their Metal layer, IOSurfaces, or on_window_should_close closure (macOS, gpui-pre 0.3.4)

Every window.remove_window() on macOS leaks the platform window's GPU state. Measured with a release build of an app that opens one window per document, repeating open-then-close five times and inspecting the process with vmmap --summary and heap after each cycle:

cycle	RSS	IOSurface	IOAccelerator (graphics)	CAMetalLayer objects alive
0 windows, after first close	112 MB	21 MB	4 MB	1
1	156 MB	43 MB	22 MB	2
2	190 MB	64 MB	39 MB	3
5	290 MB	117 MB	90 MB	6

With zero windows open, heap still lists one CAMetalLayer, two IOSurfaces and a NSWindowAuxiliary per closed window. An empty window (a single div) leaks about 25 MB per open/close cycle, so it is not content related.

Second, Window::on_window_should_close boxes the caller's closure into the platform window. Because that platform window is retained, anything the closure captures is retained too. A strong Entity<View> captured there keeps the whole view tree alive after close; in our case that was ~17 MB of decoded image per document. Downgrading to WeakEntity on the app side works around that part, but the surfaces still leak.

Repro: run any example that opens a window, close it, run heap <pid> | grep -E 'CAMetalLayer|IOSurface'; repeat.

Environment: macOS 27.0, Apple M4 Pro, gpui-kit 0.6.1 / gpui-pre 0.3.4.

Measured on https://github.com/felipefdl/openit (one window per document).