Record per-set actuals and drive the chart and volume from them

Every completed set now writes a SetEntry (reps/weight or seconds),
pre-filled from the plan by transition(to:) so the list checkbox, both
run flows, and One More all capture for free; reset clears, skip keeps
partials. The rest and finish pages show the just-done set as a pill
that opens a stepper sheet for correcting reps and weight (2.5 lb /
1.25 kg steps). The Weight Progression chart plots the top-set actual
weight and workout volume sums recorded sets, both falling back to the
plan for legacy logs via effectiveSetEntries.

Storage side of UX #3 rides along: plan weights are Double now.
Schema bumps: SplitDocument 2→3, WorkoutDocument 3→4 (a fractional
weight fails an older Int decode, and a rewrite would strip the
irreplaceable actuals), SwiftData cache 4→5. A per-log updatedAt is
reserved for the future cross-device log merge.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Shared/Persistence/WorkoutsModelContainer.swift

@@ -10,7 +10,9 @@ enum WorkoutsModelContainer {
     /// 3: added `categoryRaw` to `Exercise`.
     /// 4: removed the weight-reminder columns and `categoryRaw` from `Exercise` and
     ///    the `completed` column from `WorkoutLog`; added `machineSettings` to both.
-    static let currentSchemaVersion = 4
+    /// 5: `weight` went Int → Double on `Exercise` and `WorkoutLog`; added
+    ///    `setEntries` and `logUpdatedAt` to `WorkoutLog`.
+    static let currentSchemaVersion = 5
     private static let schemaVersionKey = "Workouts.persistenceSchemaVersion"
     private static let identityTokenKey = "Workouts.iCloudIdentityToken"