Android: Bluetooth heart-rate straps, and offline media on WorkManager

Two Android features, both already on iOS:

Offline media on WorkManager (stacksjs/hq.training#290)

• Each craft.media.keep is a unique WorkManager job. A Wi-Fi-only keep waits for an unmetered network.
• A part file left by an earlier attempt is resumed with a Range request (CraftResumableDownload, JVM-tested against a local server).
• A keep the phone can't fetch now answers at once with reason offline or cellular and queued, as on iOS.
• prune cancels jobs no longer wanted, and clear cancels every job behind a generation counter.

Bluetooth heart-rate straps (stacksjs/hq.training#240)

• enableHeartRate on Android:
  • same craft.heartRate API and craftHeartRate events as iOS (scan on 0x180D, connect, remembered strap, reconnect after a dropped link);
  • a recording kept natively, with a foreground service while it runs;
  • an emulator stand-in.
• The parser and recording store are in CraftHeartRateCore.kt.template, tested on a plain JVM against iOS's vectors.
• Gaps versus iOS:
  • a strap another app already has connected isn't listed;
  • ids are Bluetooth addresses, so a remembered strap doesn't carry between platforms;
  • Android 11 and older need location turned on to scan.

Tested so far

• Locally: packages/android index/native-shell/page-script 154/154, packages/typescript 736/736, and the Kotlin core fixture on Kotlin 2.2.10 with JDK 17.
• Not yet: the Gradle template compile runs here in CI (no local headroom), and nothing has been tried on an emulator or with a real strap.