feat(train): elevation from the barometer, its pressure kept raw (ADR 0030, D49)

11c: where a recording has GPS and the phone a barometer, the service
averages its readings over 5 s into the file's `b` lines and the
workout's new `pressure` series (tenths of a pascal). The page, Health
Connect's totals and the exported files take the fixes' altitudes from
it: the standard atmosphere's height at the pressure, levelled by the
median of GPS's altitudes less those; climbs count above 1 m. Workouts
recorded before keep GPS's altitudes; an older app skips the series.

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

@@ -1154,6 +1154,17 @@ Strava. What was found, 27 Oct:
     XML parsed, the FIT by a reader written from the specification in the tests
     (`test/support/fit_reader.dart`), not by Garmin's SDK. Not tried yet: an upload to
     Strava, intervals.icu or Garmin Connect.
+  - Built: 11c. Where a recording has GPS and the phone a barometer, the service averages
+    its readings over 5 s (`PressureAverage.kt`, tested on the JVM) into the file's `b`
+    lines and the workout's `pressure` series (`dpa`, tenths of a pascal, some 2 KB an
+    hour sealed). The page, Health Connect's totals and the files out read the fixes'
+    altitudes from it (`withBarometer`): each fix's height by the standard atmosphere at
+    the pressure then (between two readings as it went), moved by the median of GPS's
+    altitudes less those, so that its changes are the barometer's and its level GPS's;
+    its climbs count above 1 m rather than GPS's 3 m (the barometer's noise, averaged,
+    a fraction of a metre). A workout recorded before keeps GPS's altitudes. Not tried
+    on a phone yet: weather fronts over a long ride (a change of 1 hPa an hour reads as
+    some 8 m), and pockets' gusts.
 
 ## Consequences