layer-shell: recompute the exclusive-zone reservation on unmap, not just on commit

ensure_layer_initial_configure (state.rs) already recomputes the usable
monitor rect when a layer surface's exclusive zone changes, but only from
the commit pre-hook - a surface that goes away without a final commit
(zwlr_layer_surface_v1's destroy path, layer_destroyed here) never ran that
check. unmap_layer's own zone change went unnoticed.

Found live by the AGS peer session: unmapping the bar for fullscreen logged
visible=false immediately, but `srd monitors` kept reporting the bar's old
reserved_top for as long as fullscreen lasted. Harmless there only because
toggle_fullscreen targets full_geometry, which ignores the reservation
outright - but wrong for anything that reads the reserved/usable rect while
a bar is unmapped without a clean exit (a crash, not just AGS's cooperative
fullscreen hide). Same zone_before/zone_after diff ensure_layer_initial_
configure already uses, run around unmap_layer instead of arrange().

Verified: cargo build --workspace, cargo clippy -p srdwm-wayland (0 new
warnings), cargo test -p srdwm-core (111/111).

0 files changed, 0 insertions, 0 deletions