kernel: screenshot protocols + arbitrary-image decoding + input-transparent surfaces

Three additive, kernel-internal capabilities (no extension-facing signature
changes beyond the documented UiSurfaceSpec field + SurfaceElement-unrelated probes):

- Screenshots (grim): create wlr_screencopy_manager_v1 + wlr_xdg_output_manager_v1
  in init() alongside the existing compositor/data-device globals (policy-free
  plumbing; wlroots wires them to the kernel-owned outputs/renderer). The captured
  image is the standard wlr_scene_output_commit composite, so RML-composited
  documents (scene-buffer nodes) are captured correctly. wlr.hpp gains the two
  headers (static-blanking re-audited inert).

- Arbitrary raster image decode: vendor stb_image (single-header, public domain;
  warnings isolated to its own warning_level=0 TU) and extend the RMLUi
  RenderInterface LoadTexture (was uncompressed-TGA-only) to decode PNG/JPEG/BMP/
  GIF/TGA via stbi_load_from_memory (RGBA, no BGR swizzle), falling back to the
  legacy TGA path. A SubstrateSystemInterface JoinPath override stops RmlUi
  stripping the leading '/' of an absolute source during URL resolution, so both
  <img src='/abs'> and decorator: image('/abs') load the same file. Deterministic
  ui_pixel readback tests (<img> + decorator paths, red/blue PNG fixtures).

- input-transparent ui surfaces: UiSurfaceSpec gains 'bool input_transparent'
  (default false). When true the surface still composites but is skipped by the
  press-ownership hit test, so it never steals pointer/touch from windows above it
  -- required for a full-screen background (wallpaper). Deterministic seam test
  proves a transparent surface does not consume a press while an opaque one does.

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