feat(train): Days, the Loop's days and nights kept sealed (ADR 0030, D37, D38)

What the band read brings, from the bridge or the background job's
file (`LoopLines`), is kept by `DayRepository`: one sealed `day` and
`night` a date and the day's heart rate in chunks of three hours, every
ID made from the person and the date, a later read replacing what it
brought and keeping the rest, nothing rewritten unchanged; versioned,
gzipped JSON (`day_payload.dart`). `PolarLoop` reads the band as each
connection is ready (every half hour at most, or when asked), takes in
the background file as the app opens, and remembers up to which day
the days are sealed, which the next read then deletes on the band;
it keeps the background job on the band and from the right day, and
`Let the band go` stops it.

Home gains a Days tab, each day with its steps, active time, sleep and
Nightly Recharge, pulled to read the band; a day's page shows its
figures, its time in each class, its heart rate, the night's phases
and rating, Nightly Recharge and skin temperature.

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

@@ -2,8 +2,8 @@
 
 - **Status:** decided 27–28 Oct 2026, by the developer (D1–D38 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, D37, D38) is being built.
+  Connect (D29–D31) is built; the Loop's setup and keeping (7a, D34–D36) and its
+  daily data (7b, D37, D38) are 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.
@@ -666,6 +666,28 @@ Strava. What was found, 27 Oct:
   `documentation/FirstTimeUse.md` gives the same tables, without saying which class
   to take.
 
+  7b built so (28 Oct), with choices of mine for the developer to look at: home has two
+  tabs, *Workouts* and *Days*, on every device, *Days* saying where days come from
+  while there are none; the first read of a band goes four weeks back, later ones from
+  the day after those kept; the band is read as each connection is ready, at most every
+  half hour, and when the person pulls the list; a later read replaces what it brought
+  of a date (its summary, Recharge, skin temperature, heart rate, night) and keeps the
+  rest, and rewrites nothing unchanged; a day's heart rate goes in chunks by the three
+  hours each measurement began in, every row's ID made from the person and the date
+  (`https://halkyon.app/train/day/<person>/<date>`, `…/night/…`, `…/heart/<index>`); a
+  row that doesn't open on a device is left out of *Days*, as its date is sealed with
+  it; what is kept goes up to the day before the latest read's (`keptUntil`, in
+  `loop.json` with the last read's time), kept over a new setup of the same band and
+  forgotten for another; the background job runs every four hours while the battery
+  isn't low and the open app doesn't hold the band, and reads from the same day until
+  the app has kept what it wrote, so the file can hold a day more than once, the last
+  line winning; the app moves the file aside before reading it and deletes it once
+  sealed, so lines written meanwhile wait; the schedule is the phone's, for the last
+  person whose app opened with a band, and *Let the band go* stops it. Polar's
+  NONREM12 and NONREM3 are shown as light and deep sleep; the chart draws each 24/7
+  measurement's mean at its start; a night's times show in the phone's time zone. Not
+  yet tried with a Loop: whether the SDK's deletion includes its `until` day, and
+  whether the job connects to the band in the background, are for a real band.
 
 ## Consequences