phase-3: surface wl_display + wl_compositor + wl_subcompositor + EGL display to webkit engine

[mxaddict]

Sub-task of #145 (re-scoped 2026-05-17 — see comment).

apps/buffr-app currently owns winit's wl_display + wl_surface. Phase 3 needs to share those with the WebKit engine so the engine can create its own `wl_surface` as a child via wl_subsurface from the SAME wl_display (cross-client subsurface is impossible).

Scope

1. Extract Wayland handles in apps/buffr-app at startup:

   • From winit `Window::display_handle`: `RawDisplayHandle::Wayland(WaylandDisplayHandle).display` → `wl_display`.
   • From `Window::window_handle`: `RawWindowHandle::Wayland(WaylandWindowHandle).surface` → parent `wl_surface`.
   • Bind `wl_compositor` via wl_registry: `wl_display_get_registry` → listener catches `global` events → bind `wl_compositor` v6+.
   • Bind `wl_subcompositor` similarly.
   • Create EGL platform display: `eglGetPlatformDisplay(EGL_PLATFORM_WAYLAND_KHR, wl_display, NULL)` → `EGLDisplay`. (Only needed if WebKit's EGL rendering needs platform integration; verify in #145b.)

2. Cache all four handles on AppState as opaque pointers (`*mut wl_display`, etc., wrapped in `Send`-marked newtypes).

3. Pass to WebKitEngine via an extended setter. Options:

   • Extend BackendOpenOptions with new fields (couples to all backends). Too invasive.
   • Add a webkit-specific setter `engine.set_native_wayland_handles(...)` post-construction. Same pattern as `set_edit_sink`.
   • Carry through the existing `set_native_parent` (RawWindowHandle, NativeRect) by extending NativeRect with optional compositor handles. Hacky.

   Recommend option 2 — webkit-specific setter on WebKitEngine. Apps calls right after engine construction in the webkit branch.

Implementation notes

• Use the `wayland-client` Rust crate (or `wayland-sys` for raw FFI) for the wl_registry roundtrip. `wayland-sys` is lighter; `wayland-client` higher-level but pulls smithay code.
• The wl_display from winit must NOT be `wl_display_disconnect`ed by the engine — apps owns its lifetime.
• All four pointers go through to BuffrDisplayWayland (#145b) which constructs a WPEDisplay subclass that uses them.

Out of scope

• `BuffrDisplayWayland` itself (#145b).
• `BuffrViewWayland` render_buffer bridge (#145c).

Acceptance

• Apps logs the four bound handles at startup.
• WebKitEngine receives them via the setter (logged in engine).
• No build errors on X11 or headless (Wayland-only code paths gated).

Reference

• `/usr/include/wayland-client.h`
• `/usr/include/wayland-egl.h`
• `/usr/include/EGL/eglplatform.h` — EGL_PLATFORM_WAYLAND_KHR
• Parent: phase-3: create wl_subsurface from parent + attach WebKit's wl_surface as child #145, WPE WebKit backend (engine ID: webkit) — native Linux Wayland compositing #109