docs(train): the Loop's days and nights (ADR 0030, D37)

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

@@ -1,9 +1,9 @@
 # ADR 0030: Halkyon Train, what it does and where its data comes from
 
-- **Status:** decided 27–28 Oct 2026, by the developer (D1–D36 below). The shell (D12)
+- **Status:** decided 27–28 Oct 2026, by the developer (D1–D37 below). The shell (D12)
   is built; recording (D13–D28) is built on Android, iOS waiting for a Mac; Health
   Connect (D29–D31) is built; the Loop's setup and keeping (7a, D34–D36) is built, its
-  daily data (7b) comes next.
+  daily data (7b, D37) is being built.
 - **Numbering:** 0025 is held by the federation draft (stashed), 0026–0029 by the .NET
   rewrite on the local branch `rewrite-dotnet`, abandoned on 27 Oct; Train takes the next
   free number so that neither is ever read as this one.
@@ -628,6 +628,28 @@ Strava. What was found, 27 Oct:
   `verification-metadata.xml` as JitPack served them on 28 Oct, every other group
   trusted as before. Not yet tried with a Loop.
 
+- **D37 The Loop's days and nights** (recommended, the developer's; 28 Oct): what Polar's
+  SDK reads of each day (its summary: steps, distance, calories of activity, training
+  and rest, time in each activity class; its 24/7 heart rate; its sleep with phases,
+  cycles, rating and skin temperature; Nightly Recharge; skin temperature) kept as
+  Train's own models, as workouts are (over the band's raw files sealed as they come,
+  which other devices could read only with Polar's protobuf definitions, under Polar's
+  licence, and over summaries alone): one sealed, synced `day` per date (summary,
+  Nightly Recharge, skin temperature), one `night` per sleep (its phases), and the day's
+  heart rate in chunks (`day_series`, as a workout's series), each versioned JSON,
+  gzipped and sealed. The band is read while Train is open and also in the background
+  (over only while open): a WorkManager job every few hours connects and reads the days
+  since the last read into a plain file of the person's folder (app-private, as
+  `recording.txt`), which the app seals and syncs as it opens, since only the Dart side
+  holds the person's key. Train deletes nothing on the band (over deleting the days
+  kept), whose memory is taken to roll over by itself until a real band shows. Home
+  gains *Days*: each day's steps and active time with the night before's sleep and
+  Nightly Recharge, a day's page its heart rate as a chart, the night's phases and its
+  skin temperature. Steps, sleep and heart rate to Health Connect wait (D31 writes
+  workouts). D35 stays: the SDK's low-level `writeFile` could write the figures alone
+  (`PHYSDATA.BPB`) without a new user identifier, to be weighed once a real band shows
+  what a new identifier does.
+
 ## Consequences
 
 - Train's sensors are one pattern: open profiles read by the app itself, on the phone;