feat: add DeckLink input and output nodes

decklink-in:
- IDeckLinkInputCallback::VideoInputFrameArrived captures V210 frames
- Frame data stored with mutex, copied to MXL grain in process thread
- Uses mxlSleepUntil for TAI-time-based grain pacing
- Configurable: device_index (int), mode (1080i50/1080p50/etc)
- Auto-detect input format via bmdVideoInputEnableFormatDetection
- 1 output port: video_out (V210)

decklink-out:
- Reads V210 grains from MXL, schedules playback via DeckLink output
- Creates IDeckLinkMutableVideoFrame, copies grain data, schedules
- Uses ScheduledFrameCompleted callback for frame completion
- Configurable: device_index (int), mode (1080i50/1080p50/etc)
- 1 input port: video_in (V210)

Both nodes:
- Use DeckLinkAPIDispatch.cpp for CreateDeckLinkIteratorInstance
- Access pixel data via IDeckLinkVideoBuffer (latest SDK API)
- Graceful device open/close on writer/reader add/remove
- Build conditionally via DMF_BUILD_DECKLINK + DECKLINK_SDK_DIR

nodes/decklink-in/src/main.cpp

@@ -0,0 +1,6 @@
+#include <dmf-node/node_runner.hpp>
+#include "decklink_in_node.hpp"
+
+int main(int argc, char* argv[]) {
+    return dmf_node::NodeRunner::run<dmf_node::DeckLinkInNode>(argc, argv);
+}