feat: scaffold project framework with engine, node skeleton, and passthrough node

- CMake build system with vcpkg dependency management
- libdmf-node: Node interface, port definitions, WebSocket control
  server, node runner with MXL lifecycle management
- libdmf-engine: Graph model (nodes/edges CRUD, serialization),
  FlowManager (UUID + V210 NMOS flow definitions), ProcessManager
  (fork/exec node processes), ApiServer (REST API for graph control)
- Passthrough node: reads V210 grains from MXL, copies, writes to MXL
- Unit tests for Graph and FlowManager (6 cases, 21 assertions)
- MXL SDK as git submodule (symlinked from extern/mxl)

nodes/passthrough/src/passthrough_node.cpp

@@ -0,0 +1,94 @@
+#include "passthrough_node.hpp"
+
+#include <mxl/flow.h>
+#include <mxl/mxl.h>
+
+#include <spdlog/spdlog.h>
+
+#include <cstring>
+
+namespace dmf_node {
+
+void PassthroughNode::on_add_writer(const std::string& port_id, mxlFlowWriter writer) {
+    if (port_id == "video_out") {
+        writer_ = writer;
+        spdlog::info("Passthrough: writer added on video_out");
+    }
+}
+
+void PassthroughNode::on_add_reader(const std::string& port_id, mxlFlowReader reader) {
+    if (port_id == "video_in") {
+        reader_ = reader;
+        spdlog::info("Passthrough: reader added on video_in");
+    }
+}
+
+void PassthroughNode::on_remove_writer(const std::string& port_id) {
+    if (port_id == "video_out") {
+        writer_.reset();
+        spdlog::info("Passthrough: writer removed from video_out");
+    }
+}
+
+void PassthroughNode::on_remove_reader(const std::string& port_id) {
+    if (port_id == "video_in") {
+        reader_.reset();
+        spdlog::info("Passthrough: reader removed from video_in");
+    }
+}
+
+void PassthroughNode::process() {
+    if (!reader_ || !writer_) {
+        return;
+    }
+
+    mxlGrainInfo grain_info{};
+    uint8_t* payload = nullptr;
+
+    auto status = mxlFlowReaderGetGrain(*reader_, read_index_, 100000000ULL, &grain_info, &payload);
+    if (status != MXL_STATUS_OK) {
+        if (status == MXL_ERR_TIMEOUT) {
+            return;
+        }
+        if (status == MXL_ERR_OUT_OF_RANGE_TOO_LATE) {
+            mxlFlowRuntimeInfo runtime{};
+            mxlFlowReaderGetRuntimeInfo(*reader_, &runtime);
+            read_index_ = runtime.headIndex;
+            return;
+        }
+        return;
+    }
+
+    mxlGrainInfo out_grain{};
+    uint8_t* out_payload = nullptr;
+    status = mxlFlowWriterOpenGrain(*writer_, write_index_, &out_grain, &out_payload);
+    if (status != MXL_STATUS_OK) {
+        spdlog::warn("Passthrough: failed to open output grain at index {}: {}", write_index_, static_cast<int>(status));
+        read_index_++;
+        return;
+    }
+
+    auto copy_size = std::min(grain_info.grainSize, out_grain.grainSize);
+    std::memcpy(out_payload, payload, copy_size);
+
+    out_grain.validSlices = grain_info.validSlices;
+    out_grain.flags = grain_info.flags;
+    mxlFlowWriterCommitGrain(*writer_, &out_grain);
+
+    read_index_++;
+    write_index_++;
+    grains_processed_++;
+}
+
+nlohmann::json PassthroughNode::status() const {
+    return {
+        {"type", "passthrough"},
+        {"grains_processed", grains_processed_},
+        {"read_index", read_index_},
+        {"write_index", write_index_},
+        {"has_reader", reader_.has_value()},
+        {"has_writer", writer_.has_value()},
+    };
+}
+
+} // namespace dmf_node