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

CMakeLists.txt

@@ -13,6 +13,14 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
 option(DMF_BUILD_TESTS "Build tests" ON)
 option(DMF_BUILD_MXL_TOOLS "Build MXL tools (testsrc, sink)" OFF)
+option(DMF_BUILD_DECKLINK "Build DeckLink I/O nodes" ON)
+
+set(DECKLINK_SDK_DIR "" CACHE PATH "Path to Blackmagic DeckLink SDK root")
+
+if(DMF_BUILD_DECKLINK AND DECKLINK_SDK_DIR)
+    add_subdirectory(nodes/decklink-in)
+    add_subdirectory(nodes/decklink-out)
+endif()
 
 find_package(fmt CONFIG REQUIRED)
 find_package(spdlog CONFIG REQUIRED)