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

project.yml

@@ -126,7 +126,24 @@ targets:
         TARGETED_DEVICE_FAMILY: "4"
         DEVELOPMENT_ASSET_PATHS: "\"Workouts Watch App/Preview Content\""
     scheme:
-      testTargets: []
+      testTargets:
+        - Workouts Watch AppTests
+
+  # ---- watchOS unit tests (bridge apply/prune seam + watch-side helpers) ------
+  Workouts Watch AppTests:
+    type: bundle.unit-test
+    platform: watchOS
+    sources:
+      - Workouts Watch AppTests
+    dependencies:
+      - target: Workouts Watch App
+      - package: IndieSync
+    settings:
+      base:
+        PRODUCT_BUNDLE_IDENTIFIER: dev.rzen.indie.Workouts.watchkitapp.tests
+        GENERATE_INFOPLIST_FILE: true
+        SWIFT_STRICT_CONCURRENCY: complete
+        WATCHOS_DEPLOYMENT_TARGET: "26.0"
 
   # ---- watchOS widget extension (a launcher complication for the watch face) --
   Workouts Watch Widget: