docs: close Phase 0 (GO) + Phase 1 architecture design for RML compositingspike/rml-compositing

Phase 0 spike is closed GO (real-seat confirmed: input accurate through the 3D
transform, ~30fps fill-bound under the 4-window load). Records the design that
Phase 1 settles, per the user's contract decision.

- notes/rml-compositing-phase1.md (NEW): the Phase 1 design doc.
  - Contract principle: RCSS is the single source of truth for ALL layout +
    animation; C++ drives the document via a TYPED substrate API (reconciled
    with the constitution's typed-symbol rule, which governs cross-extension
    dependency discovery, not the RCSS vocabulary).
  - New kernel primitive: SurfaceElement = a LIVE analogue of Preview
    (zero-copy seq-gated import, frame-callback duty, per-subsurface tree).
  - Window layout = existing bind_list + RCSS pattern (stage-dock's, generalized).
  - Unified input-back folded into the substrate (the Element::Project mapping).
  - Damage limiting = Option B (buffer-age + wlr_damage_ring + scissor +
    set_buffer_with_damage), built properly here; scanout bypass deferred.
  - Cross-unit contract changes + Phase-2 wave plan.
  - User boundary decision: a NEW ext-window-field core extension owns the
    window field + layout; ext-xdg-shell only supplies toplevel handles +
    root wl_surface (Toplevel::scene_tree retired, wl_surface() added).
- notes/plan.md row 71: DIRECTION -> ADOPTED/GO + the Option-2 contract.
- tasks.md: slice 13 Phase 0 CLOSED; Phase 2 Wave 1 (kernel substrate) next.

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