[devmobasa]

Wayscriber's tray item advertised <prefix>/share/icons as its IconThemePath, so an AUR or other /usr install reported /usr/share/icons. Tray hosts already search that directory. Noctalia indexes the advertised path with a recursive walk on its render thread and repeats the walk for every new tray widget, so each tray drawer open walks every installed icon theme (noctalia-dev/noctalia#4338; a fix is in progress in noctalia-dev/noctalia#4341). This change stops Wayscriber from triggering it.

• The installed path is left out when it is the icons directory of a data directory that a spec-following host searches: XDG_DATA_HOME and XDG_DATA_DIRS, with the base directory spec defaults and with relative values ignored. Paths are compared after resolving symlinks. /usr, /usr/local, and ~/.local installs advertise nothing. Custom prefixes such as /opt/wayscriber or a Nix store path still advertise their own icons, which fix(tray): harden COSMIC icon fallback #160 added for those installs.
• The installed path is advertised only while the tray names its symbolic icon. Colored pixmaps need no theme path.
• WAYSCRIBER_ICON_THEME_PATH still overrides the advertised path, and an empty value advertises none. docs/CONFIG.md now documents the variable and this behavior.

The path resolution moves out of src/daemon/tray/ksni.rs into src/daemon/tray/icon_theme_path.rs. The new paths::searched_data_dirs() reads the XDG data directories.

Validation:

• Formatting and git diff --check pass.
• Workspace Clippy with all targets and -D warnings stops at an existing fetch_update deprecation in src/runtime_ui_state/controller/construction.rs, which is unchanged on main. With only that lint allowed, it passes with both all features and no default features.
• Workspace tests pass with --test-threads=1 for both all features and no default features. New tests cover a system install withheld directly and through a symlinked data directory, a custom prefix that is still advertised, and the XDG defaults, including a relative XDG_DATA_HOME.
• The C# repository checks pass.

An end-to-end check with Noctalia 5.3.0 on a headless Sway session showed the stall on every tray drawer open with the previous build installed in /usr/bin, and no stall with this branch. The icon still showed through Noctalia's normal theme lookup. The previous build with WAYSCRIBER_ICON_THEME_PATH set to an empty value gave the same result as this branch. The first open after a cold boot and niri itself were not tested.

Refs #412