waylandbridge: emit drag-and-drop drop before pointer release

[Leeestephen]

Summary

Emit the compositor-owned drag-and-drop completion before forwarding the physical
pointer-button release.

Firefox can interpret wl_pointer.button(released) as cancellation when it arrives
before wl_data_device.drop(). This results in pointerup/dragend without a DOM
drop, leaving the Wayland data offer unfinished.

Change

tr_ptr_button() now calls awl_datadev_drag_end() before forwarding the tracked
pointer release.

The paired release is still forwarded afterward because Chromium requires a balanced
pointer press/release sequence.

The resulting order is:

wl_data_device.drop()
wl_data_source.dnd_drop_performed()
wl_data_device.leave()
wl_pointer.button(..., released)
wl_data_offer.finish()
wl_data_source.dnd_finished()

Untracked releases remain consumed as before.

Testing

Tested with LibreWolf running as a native Wayland client:

• Before the change, drag-and-drop was intermittent.
• Failed attempts produced pointerup/dragend without drop.
• After the change, six consecutive controlled drags completed successfully.
• All six delivered the expected DOM drop payload.
• Each completed the Wayland offer lifecycle through wl_data_offer.finish() and
  wl_data_source.dnd_finished().

The change was also rebuilt successfully against the current upstream main.

Environment

• Anland 6.2
• Debian 13 arm64 under Droidspaces
• Xiaomi Poco F3 / Snapdragon 870
• LibreWolf 156.0-1, native Wayland
• Physical BTN_LEFT-capable mouse exposed through Android input

Scope

This PR only changes DnD event ordering. A separate frame-callback lifecycle race
discovered during testing is handled independently in fix/frame-callback-dispatch-gc.