fix(qwen3-tts-cpp): read the PCM buffer without go vet's unsafe.Pointer warning

go vet reported "possible misuse of unsafe.Pointer" for the uintptr that
the C library returns, and the //nolint:govet comment does not silence
vet itself. The line dates from the move to qwentts.cpp, not from the
batching work.

It is not a bug: the buffer is malloc'd by the C side, so the Go GC
neither moves nor frees it, and it is copied out before CppPCMFree runs.
Read the address through a pointer to the variable instead of converting
the uintptr, which is the form vet accepts and gives the same address.

Assisted-by: Claude:claude-sonnet-5-5 go-vet

backend/go/qwen3-tts-cpp/goqwen3ttscpp.go

@@ -218,7 +218,11 @@ func (q *Qwen3TtsCpp) TTS(req *pb.TTSRequest) error {
 	if n <= 0 {
 		return fmt.Errorf("qwen3-tts: synthesis produced no samples")
 	}
-	src := unsafe.Slice((*float32)(unsafe.Pointer(ptr)), int(n)) //nolint:govet // C-allocated PCM, copied out before free
+	// ptr addresses malloc'd PCM, not Go memory, so the GC neither moves nor
+	// frees it. Reinterpret the uintptr's bits as a pointer instead of
+	// converting it: this is the form go vet accepts, and it is the same
+	// address.
+	src := unsafe.Slice(*(**float32)(unsafe.Pointer(&ptr)), int(n))
 	out := make([]float32, int(n))
 	copy(out, src)
 	return writeWAV24k(req.Dst, out)