fix(launcher): only remove launcher artifacts this project created

Match the legacy entry against this checkout's desktop-run.sh instead of any
desktop-run.sh, and delete the legacy icon files only after that entry or the
old artwork proves ownership.  Quote and escape Exec arguments as the Desktop
Entry specification requires, so a checkout path with reserved characters no
longer produces an invalid entry, and escape the Path value.

Use StartupWMClass=RiotClientServices in both entries: Darling derives WM_CLASS
from -[NSProcessInfo processName], the basename of argv[0], and the launcher
starts .../RiotClientServices.  "Riot Client" is only the bundle display name,
so the checkout entry never matched the window.  Both entries now share
Name/GenericName/Comment/Icon/Keywords/Categories and a test keeps them in
sync; Terminal stays different because the checkout entry logs to
build/desktop-launch.log while the packaged entry runs the session in a
terminal.

Also drop the stray top-level icon copies, handle dangling legacy symlinks,
name the icon for assistive technology, document the alias and menu entry, and
cover the helper with disposable-HOME tests.

docs/NIX.md

@@ -91,6 +91,9 @@ darling-lol --installer
 darling-lol-shell
 ```
 
+The application menu entry is `Darling Launcher`, and `darling-launcher` is an
+alias for `darling-lol` for scripts that prefer the neutral name.
+
 If the desktop entry does not appear, ensure your desktop session includes
 `~/.nix-profile/share` in `XDG_DATA_DIRS`, then log out and back in. Non-NixOS
 systems may need this desktop-session integration after installing Nix.