ext-wallpaper: config-driven desktop wallpaper (new standard-tier unit)

A new standard-tier extension that composites a desktop background in the
'background' scene layer, below every window. Reads [wallpaper] from unbox.toml
(path / fit = cover|contain|stretch|center / color), hot-reloaded via watch_file
(drop + recreate an inline RCSS document with the values baked in -- avoids
fragile decorator data-binding). The surface is input_transparent so it never
steals clicks. Pure doctested config core; factory-only contract.

When no path is configured it falls back to a bundled default image
(assets/ext-wallpaper/default.jpg, auto-installed to the data dir), resolved to an
absolute path via $UNBOX_ASSET_DIR or the UNBOX_ASSET_DIR_DEFAULT compile define.
Degrades gracefully (color-only) if there is no GL path or the file is missing.
Sized to the primary output (multi-output is a documented gap).

Wiring (composition root): root meson subdir, host-bin installs it unconditionally
with the config path, unbox.toml gains a documented [wallpaper] sample.
(swaybg via ext-layer-shell's background band keeps working too.)

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