feat(train): recording a workout on Android (ADR 0030, D13, D16, D19, D21)

RecorderService, a foreground service of the types location and health
started from the open app, takes fixes from LocationManager's GPS and
steps from the step detector, pauses by itself (AutoPause, tested on the
JVM) and appends each sample to the person's recording file, flushed
every five seconds. MainActivity asks for location, notifications and,
for auto-pause on foot, physical activity, never background location
(D19), and bridges the service to the app.

The app follows the file's lines with RecordingBloc and shows the moving
time, the distance, the pace or speed and GPS's state; the notification
shows the time and the distance with Pause or Resume. Stop is held down;
a stopped recording is saved as a workout and its series, or discarded
after asking, and one cut off or left unsaved is offered as the app
opens. Home has the Record button where the device records.

Three choices of the implementation's are recorded under D21 for review:
rowing waits for 5f, auto-pause on foot without steps goes by GPS speed
(below 2 km/h, on above 3 km/h), and Stop's hint shows above the button.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_flutter/android/app/build.gradle.kts

@@ -47,3 +47,11 @@ kotlin {
 flutter {
     source = "../.."
 }
+
+dependencies {
+    // NotificationCompat and ServiceCompat for the recording's foreground service,
+    // the version Learn's build uses.
+    implementation("androidx.core:core:1.18.0")
+    // The recorder's pure Kotlin parts (auto-pause) tested on the JVM.
+    testImplementation("junit:junit:4.13.2")
+}