ci(smoke): fix state tier hang from bare wait on a lingering daemon

The state tier started actually running mpd in 74f0d33, which exposed a
deadlock in teardown: state.sh launches `mpd --no-daemon &` and never kills
it during the tier, and its cleanup() called smoke::stop *before* killing mpd.
smoke::stop ended in a bare `wait`, which reaps *every* background job of the
shell -- including the still-running mpd -- so it blocked until GitHub's 6h
job timeout. continue-on-error doesn't help: it catches failures, not hangs.

- lib.sh: smoke::stop now waits only on the PIDs it owns (waybar, compositor),
  so an unrelated daemon left running by a tier can't deadlock teardown.
- state.sh: cleanup() tears down mpd/pulseaudio before smoke::stop.
- smoke.yml: add timeout-minutes: 20 so a future hang fails fast instead of
  burning the default 6h runner budget.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/smoke.yml

@@ -29,6 +29,10 @@ env:
 jobs:
   smoke:
     runs-on: ubuntu-latest
+    # Safety net: the whole smoke job is minutes of work. Cap it so a hung tier
+    # (e.g. a daemon left running under a bare `wait`) fails fast instead of
+    # burning the default 6h runner budget.
+    timeout-minutes: 20
     steps:
       - uses: actions/checkout@v6