fix: don't fold non-native fullscreen self-resizes into the layout

[KorigamiK]

Problem

Apps that implement fullscreen without macOS native fullscreen (e.g. Firefox with full-screen-api.macos-native-full-screen=false) resize their own window over the whole screen with no mouse involved. handle_window_frame_changed treats that as a user resize and emits LayoutEvent::WindowResized, which rewrites the split ratios around the window — wrecking the workspace arrangement (a BSP tree collapses into everything horizontally tiled). The damage doesn't invert when the app restores its frame on exit.

Fix

Following AeroSpace's model for non-native fullscreen: in the non-drag, same-space resize path, detect frames that cover a screen (within a ±1–2px tolerance):

• Window resizes itself to cover the screen → ignore it for layout purposes (no WindowResized).
• Window shrinks back from a screen-covering frame → don't fold the transition into the ratios either; instead schedule an arrange pass so the stored layout is re-asserted.
• All other resizes behave exactly as before.

Testing

• cargo test: 403 passed, 1 failed — the failure (topology_change_clears_stale_pending_hide_target_before_next_workspace_layout) is pre-existing and also fails on clean main at 72494bf.
• Manually verified with Firefox non-native fullscreen: entering and leaving fullscreen now returns to the exact BSP arrangement from before, instead of a flattened horizontal tiling.

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