kernel: load ui surfaces from RML asset files + dev hot-reload

Externalize UI documents so RML/RCSS design changes need no C++ recompile — and,
in dev, no restart.

- UiSurfaceSpec::rml_path now actually loads the document from a file (path wins
  over rml_inline, as documented). Resolution: absolute path as-is; relative path
  against $UNBOX_ASSET_DIR, else the compile-time UNBOX_ASSET_DIR_DEFAULT (the
  install data dir), else cwd. The document URL is set so its <link> RCSS / asset
  refs resolve relative to the doc's own dir. Missing/unreadable file -> nullptr
  (degrade, never throw).
- Dev hot-reload (gated by $UNBOX_DEV): an inotify watcher integrated into the
  wl_event_loop (never blocks) watches the asset DIRS (dir-watch for IN_CLOSE_WRITE
  / IN_MOVED_TO, since editors save via temp+rename), coalesces events, and on a
  change to a surface's backing .rml/.rcss reloads the document IN PLACE:
  ClearStyleSheetCache + UnloadDocument + reload, preserving the surface's RmlUi
  context, data model and the extension's registered bind_*/bind_list* getters
  (the extension does NOT re-register), and its geometry/visibility; preview
  textures are kept. A malformed file on reload is ERROR-ISOLATED — the previous
  good document keeps rendering, one warning is logged, and a later good save
  recovers; the session never dies.
- Test seam Server::ui_reload_surface() drives reload deterministically.

ui.hpp documents rml_path + the dev hot-reload behavior. kernel 54 cases/232
assertions green on build + build-asan (incl. the UNBOX_DEV inotify path), no new
suppressions. Edits confined to packages/kernel/.

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