fix: non-blocking grain timing — use mxlGetNsUntilIndex instead of mxlSleepUntil

mxlSleepUntil blocks the entire thread including LWS poll, causing the
control server to become unresponsive and grains to be missed. Instead,
check mxlGetNsUntilIndex and skip process() if the grain isn't due yet
(>2ms away). The node loop spins with poll(1) keeping LWS responsive,
and only attempts a grain read when it's nearly due.

nodes/passthrough/src/passthrough_node.cpp

@@ -28,7 +28,6 @@ void PassthroughNode::on_add_reader(const std::string& port_id, mxlFlowReader re
         auto now = mxlGetTime();
         auto current_index = mxlTimestampToIndex(&grain_rate_, now);
         read_index_ = current_index - READ_DELAY_GRAINS;
-        delivery_deadline_ = mxlIndexToTimestamp(&grain_rate_, read_index_ + 1);
         aligned_ = true;
 
         spdlog::info("Passthrough: reader added, grain_rate={}/{}, read_index={}, delay={} grains",
@@ -55,12 +54,9 @@ void PassthroughNode::process() {
         return;
     }
 
-    auto now = mxlGetTime();
-    if (delivery_deadline_ > now) {
-        auto sleep_ns = delivery_deadline_ - now;
-        if (sleep_ns < 500000000ULL) {
-            mxlSleepUntil(delivery_deadline_);
-        }
+    auto ns_until = mxlGetNsUntilIndex(read_index_, &grain_rate_);
+    if (ns_until > 2000000ULL) {
+        return;
     }
 
     mxlGrainInfo grain_info{};
@@ -70,10 +66,9 @@ void PassthroughNode::process() {
     if (status != MXL_STATUS_OK) {
         if (status == MXL_ERR_OUT_OF_RANGE_TOO_LATE || status == MXL_ERR_OUT_OF_RANGE_TOO_EARLY) {
             auto old_index = read_index_;
-            now = mxlGetTime();
+            auto now = mxlGetTime();
             auto current_index = mxlTimestampToIndex(&grain_rate_, now);
             read_index_ = current_index - READ_DELAY_GRAINS;
-            delivery_deadline_ = mxlIndexToTimestamp(&grain_rate_, read_index_ + 1);
             spdlog::warn("Passthrough: index {} out of range ({}), realigned to {}",
                         old_index, static_cast<int>(status), read_index_);
         }
@@ -87,7 +82,6 @@ void PassthroughNode::process() {
         spdlog::warn("Passthrough: failed to open output grain at index {}: {}",
                     grain_info.index, static_cast<int>(status));
         read_index_++;
-        delivery_deadline_ = mxlIndexToTimestamp(&grain_rate_, read_index_ + 1);
         return;
     }
 
@@ -99,7 +93,6 @@ void PassthroughNode::process() {
     mxlFlowWriterCommitGrain(*writer_, &out_grain);
 
     read_index_++;
-    delivery_deadline_ = mxlIndexToTimestamp(&grain_rate_, read_index_ + 1);
     grains_processed_++;
 
     if (grains_processed_ <= 5 || grains_processed_ % 50 == 0) {