ext-xdg-shell(rml-compositing W2): add Toplevel::wl_surface() (additive)

Wave 2, additive — existing wlr_scene compositing of toplevels held intact
(the switch to surface elements is a later wave behind a flag).

- New contract: Toplevel::wl_surface() -> wlr_surface* (the root surface, ==
  xdg_toplevel->base->surface, the same surface scene_tree() hosts). A borrow
  with the same lifetime as the Toplevel (valid until on_toplevel_unmapped).
  This is the surface ext-window-field (Wave 3) passes to the kernel's
  UiSubstrate::create_surface_element(); the kernel then manages the
  subsurface/popup tree itself, so no popup/subsurface handles are exposed.
  ABI-additive: ext-keybindings/ext-stage-dock/host-bin rebuild + link unchanged.

- Fixed the ext-xdg-shell-client teardown SIGABRT. Diagnosed (not assumed): the
  abort is NOT pre-existing on baseline (seed 12e5016 passes) — it is the kernel
  Wave-1 surface-capture TEST SEAM attaching a commit Listener to every client
  wl_surface and only detaching at Server teardown, so a client destroying a
  surface mid-session trips wlroots' wl_list_empty(commit.listener_list)
  assertion. Fixed in-unit via teardown order (unmap via null-buffer commit; let
  the Server destructor reap the surface after detaching its seam). Filed a
  kernel change-request for the real fix (RAII per-surface destroy listener),
  which will also cure ext-stage-dock-glue.

ext-xdg-shell suite 2/2 green in build + build-asan, no SIGABRT, no unbox::
leak/UB. wl_surface() covered (non-null, == root surface, round-trips through
scene_tree_for()).

0 files changed, 0 insertions, 0 deletions