feat(train): music by cadence in a recording, or the metronome (ADR 0030, D48)

A workout keeps its cadence as a series of its own (steps a minute, every
10 s while moving). The Record sheet offers Music, Metronome or Off on
foot, remembered; Music once a kept track has a tempo. The recorder
measures the cadence over the last 30 s and plays a fitting track (its
tempo, half or double, within 6 %) through Media3's ExoPlayer, retuning
the track playing where it still fits, else crossfading over 3 s; a
plan's new step picks anew. A run starts at the cadence learnt from the
person's runs, which also gives a plan's steps their cadence.

Media3 1.10.1, pinned by Gradle's checksums (trusted on first download
from Google's Maven, 29 Oct 2026), for review:
  media3-common     aar 39686f54ff6baf8cbc841199afd55619533eaef2ab6cd0efc32b8a3c55dd88c3
                    module a81b2f0ebc256fe0e09f05d93c5259ade5bf44707748b9295343bee67575d9c2
  media3-container  aar a198b62c577fa434941282fbfa31eb66ff0ff308a6573edf0b5dd01f4e38445d
                    module e2a1c62c7e5d1013b906bc6ff5056db3e797cda4505e2d7d229524b24ca67a0e
  media3-database   aar 520e6f814e42a7d428db187ac073b453c0fcba0f4f5f4edd1cf33fafd2cfb740
                    module c380fea161e80d8074215a20e86e3dc2c2d825de964ca87aee66c8331f5ae252
  media3-datasource aar e22f9f779047ce951152c793af094593e7cbb52475a82d56fa4f869fd319e700
                    module 9d32d2ec6a983ee9f418bcc05777aa23eee4db37bf794f44fb66a05d6f22e963
  media3-decoder    aar a9fb2b8fa1d54b4a8d6bda98a1c655b04d59c74b3ace7e2503e9e39830f13aee
                    module 388d5226366edd1facf8eec42cb5420871a46032d545286e90eaea8454789580
  media3-exoplayer  aar 19090f829afbbb595d67618f59c212f72b433ac01dc72b47594cf42552c8ed61
                    module ba0b9ef325fa114baa0f2fbdb4caf54029606924be1999f4af02bed5f079756b
  media3-extractor  aar 3c823279356b64f4215a8c6cb1e6bfa60c533351a651786ad6fa69ad001ef1b1
                    module b3eb755d7373e4fd199ff80814005f3b303693dfb8ace16b5b9482655d49d493

Not tried on a phone yet.

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

docs/adr/0030-halkyon-train.md

@@ -1061,6 +1061,45 @@ Strava. What was found, 27 Oct:
   wait for an unmetered network) and *Delete*, which after asking unmarks every
   playlist. Not tried on a phone yet: decoding real files, how often the analysis
   lands an octave off (10c counts half and double as fitting).
+- **D48 Music in a recording** (recommended, the developer's; 29 Oct; D47's 10c):
+  - **A workout keeps its cadence** as a series of its own, `cadence` (steps a minute,
+    a sample every 10 s while moving), sealed and synced as heart rate is; an app
+    before it skips the kind, so nothing migrates (over each step's time, some 50 KB an
+    hour for detail nothing needs, and over the total alone, too little to learn
+    cadence against pace). It is Health Connect's StepsCadenceRecord, to be written
+    there later.
+  - **The Record sheet offers Music, Metronome or Off** on foot (running, walking,
+    hiking), remembered for next time; Music once some kept track has a tempo (over a
+    switch with the metronome as its fallback, which leaves no metronome to a person
+    with a library).
+  - **A change of cadence retunes, else crossfades**: within 6 % the playing track is
+    sped up or slowed down at once, beyond it the player fades over 3 s into a track
+    that fits; a plan's new step picks a fitting track at once (over playing the track
+    to its end, minutes off the beat).
+  - **Media3 1.10.1 pinned by Gradle's own checksums** (`--write-verification-metadata
+    sha256`, trusted on first download from Google's Maven, as Polar's SDK was), the
+    entries added listed in the commit for the developer to review.
+  - Built: 10c. A recording's `cadence` series is its steps counted in 10 s windows
+    from its start, a window touching a pause left out (`cadenceOf`); the steps are now
+    counted on foot whatever auto-pause listens to. The cadence a run's music starts at
+    is learnt from the person's last ten recorded runs (`cadenceModelOf`, once three of
+    them have twelve cadence samples with a precise speed each): steps a minute as a
+    line in speed, its slope kept between 0 and 25, flat where the runs' speeds barely
+    differ, and a plan's steps get the cadence of their target pace or speed from it
+    (a step's `cadence`); a walk or a hike starts at 110, a run without the model at
+    165. In the service (`MusicConductor.kt`, tested on the JVM) the cadence is the
+    steps of the last 30 s once they span 10 s, forgotten after a pause or a mark,
+    asked every 5 s; a change under 2 steps a minute does nothing. A track fits at its
+    tempo, half or double it; among those fitting, each plays once before any plays
+    again; where none fits, the nearest plays as near as 6 % allows; without tracks the
+    metronome clicks at the cadence (an AudioTrack's static loop). `MusicPlayer.kt`
+    plays with Media3's ExoPlayer, the pitch kept, the playing player holding the
+    audio focus so that the cues duck it; a new track fades in by a second player over
+    3 s; a file that won't play counts as ended. A pause by hand pauses the music,
+    auto-pause doesn't (a red light shouldn't stop the song). Music chosen where the
+    phone no longer keeps a track with a tempo shows and plays nothing. Not tried on a
+    phone yet: the crossfade, the ducking under the cues, the metronome's click, how
+    steady the cadence feels.
 
 ## Consequences