New wake-lock.ts engine (sanctioned island, dependency-injected for
tests): feature-detected navigator.wakeLock, silent no-op when
unsupported or denied, and the spec-mandated force-release-on-hide →
re-acquire-on-visible loop handled via the sentinel release event.
The player acquires on play and releases on pause (sleep-timer fades
ride the same path); the reader holds the lock for the session. Both
gate on the keep_screen_awake preference — data-keep-awake on the
player root, keepScreenAwake in readerInitExpr.
Hidden-tab playback intentionally does not keep the screen awake:
the spec forbids screen wake locks for hidden documents.
