Add weft-app-shell binary: takes <app_id> <session_id> args, connects to
zweft_shell_manager_v1 as an application window, resolves the app UI URL,
and runs a single Servo WebView in an isolated process. Prints READY to
stdout after the window is initialised so weft-appd can track the session
lifecycle.
weft-appd runtime.rs: after weft-runtime emits READY, spawn weft-app-shell
(WEFT_APP_SHELL_BIN env var) alongside it. The app shell is killed when the
session ends via abort or natural runtime exit.
weft-servo-shell: remove in-process app WebView management. The shell now
manages the system UI WebView only; all app rendering happens in dedicated
weft-app-shell processes.
