Add a watchOS test target

New Workouts Watch AppTests bundle wired into the watch scheme. Extracts
the phone-to-watch cache apply/prune into a pure, session-free
WatchCacheApplier seam and makes the HR-zone bucketing a nonisolated
static, so both can be unit-tested off the main actor without a live
WatchConnectivity session.

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

Workouts Watch App/WorkoutSessionManager.swift

@@ -142,7 +142,7 @@ final class WorkoutSessionManager: NSObject {
 
         if let maxHeartRate, let prevHR = currentHeartRate, let last = lastHRSampleDate {
             let dt = now.timeIntervalSince(last)
-            if dt > 0, dt < 60 { hrZoneSeconds[zoneIndex(for: prevHR, maxHR: maxHeartRate)] += dt }
+            if dt > 0, dt < 60 { hrZoneSeconds[Self.zoneIndex(for: prevHR, maxHR: maxHeartRate)] += dt }
         }
         if let newHR { currentHeartRate = newHR }
         lastHRSampleDate = now
@@ -151,7 +151,10 @@ final class WorkoutSessionManager: NSObject {
             .sumQuantity()?.doubleValue(for: .kilocalorie())
     }
 
-    private func zoneIndex(for hr: Double, maxHR: Double) -> Int {
+    /// Bucket an instantaneous heart rate into one of five zones (0–4) by its fraction of the
+    /// user's max HR: <60% → 0, 60–70% → 1, 70–80% → 2, 80–90% → 3, ≥90% → 4. Pure (no session
+    /// or sensor state), so it's `nonisolated static` and unit-testable off the main actor.
+    nonisolated static func zoneIndex(for hr: Double, maxHR: Double) -> Int {
         let ratio = hr / maxHR
         let thresholds = [0.6, 0.7, 0.8, 0.9]
         return thresholds.reduce(0) { $0 + (ratio >= $1 ? 1 : 0) }