fix: separate grain processing thread from LWS event loop

The fundamental issue: mxlSleepUntil/mxlSleepForNs blocks the entire
thread, making LWS unresponsive and causing control commands to hang.
Meanwhile, LWS poll(1) adds latency that makes grain timing unreliable.

Solution: run node->process() in a dedicated thread that can freely use
mxlSleepUntil for precise grain timing, while the main thread runs
control_server->poll(10) for LWS event handling.

Passthrough now uses mxlSleepUntil(deadline) matching the mxl-gst-sink
Cursor pattern, with 2-grain read delay for buffering.

nodes/passthrough/src/passthrough_node.cpp

@@ -51,13 +51,12 @@ void PassthroughNode::on_remove_reader(const std::string& port_id) {
 
 void PassthroughNode::process() {
     if (!reader_ || !writer_ || !aligned_) {
+        std::this_thread::sleep_for(std::chrono::milliseconds(1));
         return;
     }
 
-    auto ns_until = mxlGetNsUntilIndex(read_index_, &grain_rate_);
-    if (ns_until > 2000000ULL) {
-        return;
-    }
+    auto deadline = mxlIndexToTimestamp(&grain_rate_, read_index_ + 1);
+    mxlSleepUntil(deadline);
 
     mxlGrainInfo grain_info{};
     uint8_t* payload = nullptr;