fix: verify demo bundle freshness for smokes too

Golden runs, benchmarks and documentation captures each called
assertFreshDemoBundle; the smoke launcher never did, so all ~128 smokes could
silently test a stale demo/srv/assets bundle. On #234 that cost a round of
analysis: three assertions went red and a fourth went green, because the old
code was wrong in two places that agreed with each other, and a mixed result
reads as a logic defect rather than a stale artefact.

launch() now runs the check once for every smoke, against the repository root
rather than the serving root — demo/srv has no src/** to fingerprint.
HP_ALLOW_STALE_BUNDLE=1 skips it for debugging and warns out loud, because a
guard that says nothing when it steps aside is the silent success this project
keeps removing. A mutation entry proves the call cannot quietly disappear.

Issue: #236
User-Visible: no

scripts/mutation-gate.mjs

@@ -697,8 +697,8 @@ export const MUTANTS = [
       + 'changing the separate room-fill gradient',
     patches: [{
       file: 'src/device-presentation.ts',
-      find: '  if (lqi <= 40) return \\'low\\';',
-      replace: '  if (lqi < 40) return \\'low\\';',
+      find: '  if (lqi <= 40) return \'low\';',
+      replace: '  if (lqi < 40) return \'low\';',
     }],
   },
   {