Add WorkoutsTests unit test target

No unit test target existed (testTargets: [] in scheme). Adds a
bundle.unit-test target wired into the Workouts scheme, with a smoke test
that round-trips SplitDocument through IndieSync's DocumentCoder — the
same codec DocumentFileStore uses for every iCloud document.

WorkoutsTests/SplitDocumentCodableTests.swift

@@ -0,0 +1,45 @@
+import Foundation
+import Testing
+import IndieSync
+@testable import Workouts
+
+/// Round-trips `SplitDocument` through `DocumentCoder` — the exact JSON codec
+/// (ISO-8601 dates, sorted keys, pretty-printed) `DocumentFileStore` uses to
+/// write and read every document in this app's iCloud container.
+struct SplitDocumentCodableTests {
+    /// A whole-second epoch so ISO-8601 round-trips (no fractional-second loss).
+    private static let stableTimestamp = Date(timeIntervalSince1970: 1_700_000_000)
+
+    @Test func encodeDecodeRoundTrip() throws {
+        let exercise = ExerciseDocument(
+            id: ULID.make(),
+            name: "Bench Press",
+            order: 0,
+            sets: 4,
+            reps: 10,
+            weight: 135,
+            loadType: 0,
+            durationSeconds: 0,
+            weightLastUpdated: Self.stableTimestamp,
+            weightReminderWeeks: 4
+        )
+        let original = SplitDocument(
+            schemaVersion: SplitDocument.currentSchemaVersion,
+            id: ULID.make(),
+            name: "Push Day",
+            color: "blue",
+            systemImage: "dumbbell.fill",
+            order: 0,
+            createdAt: Self.stableTimestamp,
+            updatedAt: Self.stableTimestamp,
+            exercises: [exercise],
+            activityType: nil
+        )
+
+        let data = try DocumentCoder.encode(original)
+        let decoded = try DocumentCoder.decode(SplitDocument.self, from: data)
+
+        #expect(decoded == original)
+        #expect(decoded.relativePath == "Splits/\(original.id).json")
+    }
+}