Save workouts to Apple Health with live watch metrics and a post-workout summary

Watch sessions now run an HKLiveWorkoutBuilder: heart rate and active
energy are collected live (shown in an in-workout HUD), saved to Health
as a real HKWorkout on completion, and carried back to the phone as
WorkoutMetrics on the workout document. Phone-only workouts get an
estimated Health workout (MET x bodyweight x duration) after a 10s
debounce that a late-arriving watch session cancels; a launch sweep
backfills workouts completed within the last day. Dedupe is keyed on
metrics.healthKitWorkoutUUID plus the metrics source.

Splits gain an activity type (strength, functional, HIIT, core, cardio,
cycling) that categorizes the Health workout and picks the MET value.
A post-workout summary sheet (duration, calories, avg/max HR, HR zones,
total volume) fills in live and is also shown on completed workouts.
Weights can now display in lb or kg (display-only relabel), synced to
the watch over the existing application context.

WorkoutDocument schema 1->2 (metrics are irreplaceable, so older apps
must quarantine rather than strip them); cache schema 2 rebuilds the
SwiftData store with the new metric columns. Deleting a workout in the
app intentionally leaves its Health record in place.

Shared/Persistence/WorkoutsModelContainer.swift

@@ -6,7 +6,8 @@ import SwiftData
 /// safe; `SyncEngine` repopulates it from the container on next launch.
 enum WorkoutsModelContainer {
     /// Bump whenever the cache schema changes — wipes and rebuilds from files.
-    static let currentSchemaVersion = 1
+    /// 2: added health-metric columns to `Workout` and `activityTypeRaw` to `Split`.
+    static let currentSchemaVersion = 2
     private static let schemaVersionKey = "Workouts.persistenceSchemaVersion"
     private static let identityTokenKey = "Workouts.iCloudIdentityToken"