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.hpp

@@ -0,0 +1,45 @@
+#pragma once
+
+#include <dmf-node/node.hpp>
+
+#include <mxl/flow.h>
+
+#include <unordered_map>
+#include <optional>
+
+namespace dmf_node {
+
+class PassthroughNode : public Node {
+public:
+    PassthroughNode() = default;
+
+    std::string type() const override { return "passthrough"; }
+
+    std::vector<PortDef> input_ports() const override {
+        return {{"video_in", PortDirection::Input, MediaType::VideoV210}};
+    }
+
+    std::vector<PortDef> output_ports() const override {
+        return {{"video_out", PortDirection::Output, MediaType::VideoV210}};
+    }
+
+    void configure(const nlohmann::json& /*params*/) override {}
+
+    void on_add_writer(const std::string& port_id, mxlFlowWriter writer) override;
+    void on_add_reader(const std::string& port_id, mxlFlowReader reader) override;
+    void on_remove_writer(const std::string& port_id) override;
+    void on_remove_reader(const std::string& port_id) override;
+
+    void process() override;
+    nlohmann::json status() const override;
+
+private:
+    std::optional<mxlFlowReader> reader_;
+    std::optional<mxlFlowWriter> writer_;
+    mxlInstance mxl_instance_ = nullptr;
+    uint64_t read_index_ = 0;
+    uint64_t write_index_ = 0;
+    uint64_t grains_processed_ = 0;
+};
+
+} // namespace dmf_node