ext-window-field(rml-compositing W3): toplevels as RCSS surface elements

NEW core unit (id "window-field", depends_on "xdg-shell"), installed by host-bin
ONLY under --rml-compositing / UNBOX_RML_COMPOSITING (default off; the classic
wlr_scene path stays the default until RML compositing is signed off on hardware).

It composites application toplevels as RCSS surface elements inside ONE ui
surface (the window field) instead of as wlr_scene nodes:
- activate(): fetch ext-xdg-shell Service (typed), create the window-field
  UiSurface at SceneLayer::normal sized to the primary output, subscribe to
  on_output_added + on_toplevel_mapped/unmapped/focused.
- on map: Toplevel::wl_surface() -> UiSubstrate::create_surface_element(); the
  toplevel is Toplevel::hide()'d out of wlr_scene (the surface element is now the
  sole compositor of its pixels; the substrate drives its frame callbacks);
  appended to bind_list("wins") (live_uri = source_uri(), focused, title, app_id).
- on unmap: drop the SurfaceElement + row. on focus: flip the focused flag.
- Layout/animation are RCSS (assets/ext-window-field/field.{rml,rcss}): a wrapping
  flex field where the focused window dominates and the rest tile below, animated
  on a transition. No geometry from C++ (the user's contract decision).
- Keyboard focus stays ext-xdg-shell's job; pointer/touch input-back is automatic
  in the kernel; this unit wires no seat calls.

Wiring (orchestrator): root meson subdir, host-bin install behind the flag +
ext_window_field_dep. Headless glue test (40 assertions): map tracks the window +
drives hide(), 2nd map, focus flip via the Alt+Tab path, unmap removes it; no-GL
degrade asserted. build + build-asan green; host-bin links; no regressions.

Known gaps (change-requests): click-to-focus a BACKGROUND window needs a kernel
"surface-element pressed -> notify owner" hook (next); wallpaper needs
ext-layer-shell surface exposure (Wave 3b).

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