v0.4.0

v0.4.0 - Android battery / responsiveness / slow-device overhaul

Decode workers now wake the frame loop themselves, so on-demand rendering is
real on both platforms: an open book, the library, and a backgrounded app all
idle at zero CPU. The whole-volume preview tier moved into a lowest-priority
pool queue (2-worker cap, HQ-reserved lane, per-stride rebuilds). Device
tiers scale the decode budget on weaker hardware with a new Performance
setting (Auto/Battery saver/Balanced/Performance) that applies instantly;
flagship behavior is unchanged. Book opens, sibling scans, the info overlay,
and position saves are all async/atomic on Android; backgrounding parks the
decode pool and resume reattaches the parked book in milliseconds instead of
seconds. KEEP_SCREEN_ON only holds while reading (via a JNI bridge - the
android-activity set_window_flags wrapper deadlocks the event loop). LQ JPEG
thumbnails decode IDCT-scaled. Fat LTO release builds.