Optimize AAB Size

Target: <20MB download for cellular install.

Measure baseline

bundletool get-size total --apks=app.apks --dimensions=ABI,LANGUAGE,SCREEN_DENSITY

Top size offenders

1. Native binaries (~25MB):
   • React Native core
   • Reanimated, Skia (if added)
   • Hermes (already enabled by default)
   • expo modules