Harden watch sync against schema mismatches and surface log-row settings

The watch now re-applies the phone's application context once activation
completes (real hardware activates asynchronously, so the eager launch
read sees an empty context), and a state push that fails to decode —
a phone/watch build running different document schemas — is logged and
skipped instead of pruning the cache against a bogus empty set. Workout
log rows offer the machine-settings editor for any machine-based library
exercise, not just logs that already carry settings.

Claude-Session: https://claude.ai/code/session_01LEoff8bXGBS83tK1c55Mf7

Shared/Connectivity/WCPayload.swift

@@ -39,14 +39,18 @@ enum WCPayload {
         return dict
     }
 
-    static func decodeSplits(_ dict: [String: Any]) -> [SplitDocument] {
+    /// `nil` means the payload carried splits that failed to decode (a schema mismatch
+    /// between the two builds) — distinct from an absent key or a legitimately empty
+    /// list, so the receiver can surface it instead of silently applying nothing.
+    static func decodeSplits(_ dict: [String: Any]) -> [SplitDocument]? {
         guard let data = dict[splitsKey] as? Data else { return [] }
-        return (try? DocumentCoder.decoder.decode([SplitDocument].self, from: data)) ?? []
+        return try? DocumentCoder.decoder.decode([SplitDocument].self, from: data)
     }
 
-    static func decodeWorkouts(_ dict: [String: Any]) -> [WorkoutDocument] {
+    /// See `decodeSplits` — `nil` is a decode failure, not an empty list.
+    static func decodeWorkouts(_ dict: [String: Any]) -> [WorkoutDocument]? {
         guard let data = dict[workoutsKey] as? Data else { return [] }
-        return (try? DocumentCoder.decoder.decode([WorkoutDocument].self, from: data)) ?? []
+        return try? DocumentCoder.decoder.decode([WorkoutDocument].self, from: data)
     }
 
     static func decodeRestSeconds(_ dict: [String: Any]) -> Int? { dict[restSecondsKey] as? Int }