feat: extend BindPayload with optional loop field; add setPlayMode service

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Tests/DuooomiBleSDKTests/PlayModeTests.swift

@@ -50,3 +50,34 @@ final class DeviceInfoLoopTests: XCTestCase {
         XCTAssertNil(info.loop)
     }
 }
+
+final class BindPayloadEncodingTests: XCTestCase {
+    private func encode(_ payload: BindPayload) throws -> [String: Any] {
+        let data = try JSONEncoder().encode(payload)
+        return try JSONSerialization.jsonObject(with: data) as? [String: Any] ?? [:]
+    }
+
+    func testBindPayloadWithoutLoopOmitsKey() throws {
+        let payload = BindPayload(type: 0x0F, userId: "u1", loop: nil)
+        let dict = try encode(payload)
+        XCTAssertEqual(dict["type"] as? Int, 0x0F)
+        XCTAssertEqual(dict["userId"] as? String, "u1")
+        XCTAssertNil(dict["loop"], "loop key MUST be omitted when nil (not encoded as null)")
+        XCTAssertEqual(dict.count, 2)
+    }
+
+    func testBindPayloadWithLoopIncludesValue() throws {
+        let payload = BindPayload(type: 0x0F, userId: "u1", loop: 1)
+        let dict = try encode(payload)
+        XCTAssertEqual(dict["type"] as? Int, 0x0F)
+        XCTAssertEqual(dict["userId"] as? String, "u1")
+        XCTAssertEqual(dict["loop"] as? Int, 1)
+        XCTAssertEqual(dict.count, 3)
+    }
+
+    func testBindPayloadWithLoopZeroIncludesValue() throws {
+        let payload = BindPayload(type: 0x0F, userId: "u1", loop: 0)
+        let dict = try encode(payload)
+        XCTAssertEqual(dict["loop"] as? Int, 0)
+    }
+}