fix: use mxlSleepUntil delivery deadline pattern for grain timing

Match the mxl-gst-sink pattern: sleep until the next grain's delivery
deadline using mxlSleepUntil, then read with a short 5ms timeout.
Uses 2-grain read delay (40ms at 50fps) for buffering headroom.

Previous approach of mxlSleepForNs + 20ms GetGrain timeout caused the
passthrough to fall behind: each iteration took ~25ms (poll+sleep+read),
missing grains and perpetually chasing headIndex via realign.

nodes/passthrough/src/passthrough_node.cpp

@@ -25,9 +25,14 @@ void PassthroughNode::on_add_reader(const std::string& port_id, mxlFlowReader re
         mxlFlowReaderGetConfigInfo(*reader_, &config);
         grain_rate_ = config.common.grainRate;
 
-        realign();
-        spdlog::info("Passthrough: reader added on video_in, grain_rate={}/{}, read_index={}",
-                     grain_rate_.numerator, grain_rate_.denominator, read_index_);
+        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",
+                     grain_rate_.numerator, grain_rate_.denominator, read_index_, READ_DELAY_GRAINS);
     }
 }
 
@@ -45,32 +50,32 @@ void PassthroughNode::on_remove_reader(const std::string& port_id) {
     }
 }
 
-void PassthroughNode::realign() {
-    auto now = mxlGetTime();
-    auto current_index = mxlTimestampToIndex(&grain_rate_, now);
-    constexpr int64_t read_delay_grains = 1;
-    read_index_ = current_index - read_delay_grains;
-    aligned_ = true;
-}
-
 void PassthroughNode::process() {
     if (!reader_ || !writer_ || !aligned_) {
         return;
     }
 
-    auto wait_ns = mxlGetNsUntilIndex(read_index_, &grain_rate_);
-    if (wait_ns > 0 && wait_ns < 500000000ULL) {
-        mxlSleepForNs(wait_ns);
+    auto now = mxlGetTime();
+    if (delivery_deadline_ > now) {
+        auto sleep_ns = delivery_deadline_ - now;
+        if (sleep_ns < 500000000ULL) {
+            mxlSleepUntil(delivery_deadline_);
+        }
     }
 
     mxlGrainInfo grain_info{};
     uint8_t* payload = nullptr;
 
-    auto status = mxlFlowReaderGetGrain(*reader_, read_index_, 20000000ULL, &grain_info, &payload);
+    auto status = mxlFlowReaderGetGrain(*reader_, read_index_, 5000000ULL, &grain_info, &payload);
     if (status != MXL_STATUS_OK) {
         if (status == MXL_ERR_OUT_OF_RANGE_TOO_LATE || status == MXL_ERR_OUT_OF_RANGE_TOO_EARLY) {
-            spdlog::warn("Passthrough: index {} out of range ({}), realigning", read_index_, static_cast<int>(status));
-            realign();
+            auto old_index = read_index_;
+            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_);
         }
         return;
     }
@@ -79,8 +84,10 @@ void PassthroughNode::process() {
     uint8_t* out_payload = nullptr;
     status = mxlFlowWriterOpenGrain(*writer_, grain_info.index, &out_grain, &out_payload);
     if (status != MXL_STATUS_OK) {
-        spdlog::warn("Passthrough: failed to open output grain at index {}: {}", grain_info.index, static_cast<int>(status));
-        read_index_ = grain_info.index + 1;
+        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;
     }
 
@@ -91,7 +98,8 @@ void PassthroughNode::process() {
     out_grain.flags = grain_info.flags;
     mxlFlowWriterCommitGrain(*writer_, &out_grain);
 
-    read_index_ = grain_info.index + 1;
+    read_index_++;
+    delivery_deadline_ = mxlIndexToTimestamp(&grain_rate_, read_index_ + 1);
     grains_processed_++;
 
     if (grains_processed_ <= 5 || grains_processed_ % 50 == 0) {

nodes/passthrough/src/passthrough_node.hpp

@@ -33,12 +33,13 @@ public:
     nlohmann::json status() const override;
 
 private:
-    void realign();
+    static constexpr int64_t READ_DELAY_GRAINS = 2;
 
     std::optional<mxlFlowReader> reader_;
     std::optional<mxlFlowWriter> writer_;
     mxlRational grain_rate_{50, 1};
     uint64_t read_index_ = 0;
+    uint64_t delivery_deadline_ = 0;
     uint64_t grains_processed_ = 0;
     bool aligned_ = false;
 };