Ride on Android: the same BLE stack, over JNI

G-4 said port to Android without rewriting the core, and nothing in
crates/core, crates/ble or crates/fit needed touching (NFR-5) — the
Android work is two files of glue and a Gradle project.

btleplug's Android backend is a hybrid crate: the GATT work happens in
Java and Rust drives it over JNI. `platform::init` has to run once with a
JNIEnv, and it cannot come from Rust's own startup — JNI resolves classes
with the calling thread's class loader, and a thread Rust spawned has
only the bootstrap loader. So MainActivity.onCreate calls into
src/android.rs, before super.onCreate: TauriActivity's super chain
synchronously starts the thread that runs `run()`, which builds AppState
and starts scanning while we are still in onCreate. Lose that race and
droidplug's global_adapter() — an `expect` — panics inside the scan task,
silently, for the life of the process.

Failing soft here is not enough for the same reason, so init sets a READY
flag and devices.rs asks before every call in. Bluetooth switched off at
launch then reads as an ordinary "no adapter", which the connection
screen already knows how to show, and onResume retries so switching it on
and coming back works.

The Java half is not a maven dependency. Upstream tells you to publish a
0.1.1-SNAPSHOT artifact to mavenLocal by hand, which no CI runner can
reproduce and which drifts from the crate silently — the failure is a
NoSuchMethodError at the first scan, not a build error. Instead
sync-android-sources.sh lifts the classes out of the btleplug and
jni-utils crate sources at exactly the versions in Cargo.lock, so a
mismatch is impossible by construction.

gen/ stays generated and untracked, so everything hand-written lives in
src-tauri/android/ and is copied back after each `tauri android init`.
check-android-sources.sh fails the build if a source exists only under
gen/ or differs from its tracked copy: both are files git has never seen
and the next init deletes, and the resulting APK builds, installs, and
behaves as though they were never written.

Permissions are split at API 31, because asking for one the platform does
not know is a permanent denial. neverForLocation on BLUETOOTH_SCAN is a
promise we can keep honestly: every scan filters by service UUID, so no
location permission is needed on Android 12+.

Also: tracing to logcat, since Android has no stdout and the default
writer drops every line into a closed fd.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src-tauri/android/app/proguard-bikecontrol.pro

@@ -0,0 +1,24 @@
+# Keep rules for the BLE stack.
+#
+# Every class below is reached only from Rust, over JNI, by name. R8 sees no
+# reference to any of it and would strip or rename the lot — and the failure is
+# invisible until the first scan on a *release* build, where btleplug's
+# `platform::init` throws ClassNotFound and the app finds no trainer. Debug
+# builds are unminified, so this cannot be caught by testing locally.
+
+# btleplug's Android backend. `register_native_methods` binds by exact method
+# name and signature, and the exception classes are looked up so the Rust side
+# can map them onto btleplug::Error.
+-keep class com.nonpolynomial.btleplug.** { *; }
+
+# jni-utils: the Future/Stream/Waker plumbing btleplug's Java calls back into.
+-keep class io.github.gedgygedgy.rust.** { *; }
+
+# The Rust side calls these on the Android framework classes it is handed.
+-keepclassmembers class * extends android.bluetooth.BluetoothGattCallback { *; }
+-keepclassmembers class * extends android.bluetooth.le.ScanCallback { *; }
+
+# Our own JNI entry point, called from MainActivity as an `external fun`.
+-keepclasseswithmembernames class paris.tourolle.bikecontrol.MainActivity {
+    native <methods>;
+}