docs: plan RML compositing direction + roadmap re-scope

Major architecture direction (gated by a spike): make the RMLUi substrate the
content compositor — toplevels, layer-shell clients (incl. wallpaper), and UI
chrome become RML elements backed by live, shared GL textures, with layout,
animation, and 3D effects expressed in RCSS. wlroots stays the foundation +
hardware cursor plane + a deferred fullscreen-video scanout bypass. Lost
wlr_scene damage/scanout is mitigated by our own dirty-gated rendering (not a
RMLUi built-in) + the deferred scanout bypass.

- notes/rml-compositing.md: full architecture, the de-risking already done
  (shared EGLDisplay; RmlUi transform-aware hit-testing + RTT/filters/shaders;
  slice 3 + stage dock proved dmabuf/EGLImage), division of labour, perf
  posture, Phase 0 spike acceptance criteria, Phase 1/2, naming proposal.
- notes/plan.md §2: decision row (reopens the compositing half of row 51) with
  rationale + explicit spike fallback.
- tasks.md: slice 13 (RML compositing spike) = ACTIVE core; slice 7 (tiling)
  DEFERRED behind it; "Now" leads with the pivot.

Roadmap re-scope (earlier this session):
- notes/tiling-spec.md: baseline tiling design (primary/stack, configurable
  side + new-window insertion, hot-reloadable [tiling] config) — deferred.
- notes/tiling-layouts-reference.md: dwm layout catalogue for later.
- notes/status-bar-home-screen.md: status bar + iPad-style home screen ideas
  (replace the cut window-list taskbar); tasks.md slices 6 (cut), 11, 12.

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