Slice 4: extension host + typed bus; xdg-shell/layer-shell extracted to core extensions

The kernel now names NO concrete feature. It owns: the extension host
(install/topological activate, missing-dep/cycle = startup error), the
typed Event/Filter bus (error-isolated: a throwing extension is disabled,
never the session; RAII Subscriptions), the Host API (per-extension
facade: borrows, scene layers, event catalogue, typed services), the
public RAII Listener, and a typed surface→scene-tree registry
(Host::host_surface/scene_tree_for) that replaced the untyped
wlr_surface.data convention both extensions flagged.

- ext-xdg-shell (core): toplevel/popup lifecycle, focus-on-map,
  click/tap-to-focus, pointer/touch routing incl. button+axis (the
  kernel only moves the cursor and emits — a contract-doc lie caught by
  user hands-on), interactive move/resize via pure GrabMachine (fixes
  the request-arrives-after-release race: grab requires request ∧
  button-down, release always ends it), Alt+F1 cycle,
  Ctrl+Alt+Backspace terminate (labwc's default A-Escape=Exit killed
  the dev session once — never again; see nested-run skill).
- ext-layer-shell (core): wlr-layer-shell v1 (proto v5) for external
  clients; pure doctest-hard arrangement core; fuzzel verified visually
  nested (fix: seed outputs from output_layout at activate — events-only
  tracking missed pre-activation outputs; plus a scene-node double-free).
- First protocol codegen: vendored wlr-layer-shell XML + wayland-scanner
  server-header propagated through kernel_dep; wlr.hpp grew a
  namespace→_namespace keyword fix for the generated header.
- Glossary: 'scene layer' (user-approved). New rules earned:
  parallel-wave-builds, contract-docs.
- User hands-on verified: typing, click-to-focus, drag-select, scroll,
  titlebar drag-move (slow + flick), Alt+F1, fuzzel + arrows, touch tap,
  Ctrl+Alt+Backspace. 68 doctest cases green, ASan/UBSan clean (our
  code), idle RSS ≈73 MiB.

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