Add generic watch-tracked Cardio exercise and starter split

A single "Cardio" library entry (running-in-place figure, duration-logged)
plus a matching Cardio starter split tagged .cardio (HealthKit .mixedCardio),
so the Apple Watch records the real aerobic workout while the phone logs the
time — no per-machine cardio entries or schema changes.

Claude-Session: https://claude.ai/code/session_012qw2itfzKyEJ1HpsFt8Ex4

WorkoutsTests/SeedLibraryTests.swift

@@ -11,16 +11,16 @@ import IndieSync
 /// both the raw document and a SwiftData cache round-trip.
 struct SeedLibraryTests {
 
-    @Test func catalogLoadsTwelveSeedsInOrder() {
+    @Test func catalogLoadsThirteenSeedsInOrder() {
         let seeds = SeedLibrary.seeds
-        #expect(seeds.count == 12)
-        #expect(seeds.map(\.doc.order) == Array(0...11))
+        #expect(seeds.count == 13)
+        #expect(seeds.map(\.doc.order) == Array(0...12))
         #expect(seeds.map(\.doc.name) == [
             "Upper Body", "Core", "Lower Body", "Bodyweight Core",
             "Full Body Machines", "Free Weight Basics",
             "Upper Body Warm-Up", "Lower Body Warm-Up",
             "Morning Wake-Up", "Morning Mobility",
-            "Full Body Stretch", "Evening Stretch",
+            "Full Body Stretch", "Evening Stretch", "Cardio",
         ])
     }
 
@@ -44,6 +44,7 @@ struct SeedLibraryTests {
             "01DXF6DT00QHJ9G4SDMEX64NZD", // Morning Mobility
             "01DXF6DT003PRJB3FGYSSGTQ5X", // Full Body Stretch
             "01DXF6DT00M5YRFPHJQ2FS2B9P", // Evening Stretch
+            "01DXF6DT00BXJ6N63ZBX9M9VNH", // Cardio
         ])
     }