runtime.rs — process lifecycle manager:
- supervise(session_id, app_id, registry): spawns the weft-runtime child
process identified by WEFT_RUNTIME_BIN env var. If unset, logs debug
and returns immediately (no-op until runtime binary is available).
- Child process invoked as: <WEFT_RUNTIME_BIN> <app_id> <session_id>
with stdout/stderr piped, stdin closed.
- wait_for_ready(): reads stdout line-by-line; returns Ok(()) on first
line matching 'READY'; returns Err if stdout closes without it.
- 30-second READY_TIMEOUT via tokio::time::timeout; on expiry, kills
the child and transitions session to Stopped.
- On success: sets session state to Running, broadcasts AppReady to all
connected WebSocket clients via registry broadcast channel.
- drain_stderr(): async task that forwards child stderr lines to tracing
at WARN level for observability.
- On process exit: sets session state to Stopped regardless of exit code.
main.rs — wiring:
- SessionRegistry now owns broadcast::Sender<Response>; Default creates
the channel internally. Added set_state(), subscribe(), broadcast()
methods. Removed standalone broadcast_tx from run(); WS handlers
subscribe via registry.lock().await.subscribe().
- dispatch::LaunchApp spawns a tokio task calling runtime::supervise
immediately after creating the session. supervise is a no-op when
WEFT_RUNTIME_BIN is unset, so existing tests are unaffected.
Cargo.toml: added tokio 'process' and 'time' features.
