docs: add build and test instructions

README.md

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+# DMF Studio
+
+Node-based visual production platform for the Dynamic Media Facility architecture.
+
+## Prerequisites
+
+- CMake 3.24+
+- C++20 compiler (GCC 12+, Clang 15+)
+- vcpkg (at `~/vcpkg` or set `CMAKE_TOOLCHAIN_FILE`)
+- GStreamer (for MXL test tools only)
+
+## Build
+
+```bash
+# Configure (from project root)
+cmake -B build \
+  -DCMAKE_BUILD_TYPE=Debug \
+  -DCMAKE_TOOLCHAIN_FILE=$HOME/vcpkg/scripts/buildsystems/vcpkg.cmake \
+  -DBUILD_SHARED_LIBS=OFF \
+  -DBUILD_DOCS=OFF \
+  -DBUILD_TESTS=OFF \
+  -DBUILD_TOOLS=OFF \
+  -DBUILD_UTILS=OFF
+
+# Build all
+cmake --build build -j$(nproc)
+```
+
+Binaries end up in:
+- `build/engine/dmf-studio-engine`
+- `build/nodes/passthrough/dmf-node-passthrough`
+
+## Rebuild after changes
+
+```bash
+# Full rebuild
+cmake --build build -j$(nproc)
+
+# Rebuild only one target (faster)
+cmake --build build -j$(nproc) --target dmf-node-passthrough
+cmake --build build -j$(nproc) --target dmf-studio-engine
+```
+
+## Clean rebuild
+
+```bash
+rm -rf build
+# Then re-run the configure + build steps above
+```
+
+## Run unit tests
+
+```bash
+build/tests/dmf-test-graph
+```
+
+## Test with MXL
+
+### 1. Create MXL domain
+
+```bash
+mkdir -p /tmp/dmf-mxl
+```
+
+### 2. Start MXL test source (writes V210 video flow)
+
+Create a flow config file:
+
+```bash
+cat > /tmp/v210_50p.json << 'EOF'
+{
+  "id": "a0000001-0000-0000-0000-000000000001",
+  "description": "DMF Studio test video flow",
+  "format": "urn:x-nmos:format:video",
+  "label": "DMF Studio Test Video",
+  "tags": {
+    "urn:x-nmos:tag:grouphint/v1.0": ["dmf-studio:Video"]
+  },
+  "media_type": "video/v210",
+  "grain_rate": {"numerator": 50, "denominator": 1},
+  "frame_width": 1920,
+  "frame_height": 1080,
+  "interlace_mode": "progressive",
+  "colorspace": "BT709",
+  "components": [
+    {"name": "Y", "width": 1920, "height": 1080, "bit_depth": 10},
+    {"name": "Cb", "width": 960, "height": 1080, "bit_depth": 10},
+    {"name": "Cr", "width": 960, "height": 1080, "bit_depth": 10}
+  ]
+}
+EOF
+```
+
+Start test source (needs GStreamer + MXL tools built separately):
+
+```bash
+mxl-gst-testsrc -d /tmp/dmf-mxl -v /tmp/v210_50p.json --pattern smpte &
+```
+
+Check active flows:
+
+```bash
+mxl-info --domain /tmp/dmf-mxl
+```
+
+### 3. Start passthrough node
+
+```bash
+build/nodes/passthrough/dmf-node-passthrough \
+  --node-id pass1 \
+  --control-port 9100 \
+  --mxl-domain /tmp/dmf-mxl
+```
+
+Options:
+- `--node-id, -n` — Unique node instance ID (required)
+- `--control-port, -p` — WebSocket control port (required)
+- `--mxl-domain, -d` — MXL domain path (default: `/dev/shm/mxl`)
+- `--config, -c` — Node config as JSON string
+
+### 4. Start engine
+
+```bash
+export DMF_STUDIO_BIN_DIR=build/nodes/passthrough
+build/engine/dmf-studio-engine --port 8080
+```
+
+### 5. Control via REST API
+
+```bash
+# Add nodes
+curl -X POST http://localhost:8080/api/graph/nodes \
+  -H "Content-Type: application/json" \
+  -d '{"type":"passthrough","id":"pass1"}'
+
+curl -X POST http://localhost:8080/api/graph/nodes \
+  -H "Content-Type: application/json" \
+  -d '{"type":"passthrough","id":"pass2"}'
+
+# Connect nodes (creates MXL flow between them)
+curl -X POST http://localhost:8080/api/graph/edges \
+  -H "Content-Type: application/json" \
+  -d '{"from_node":"pass1","from_port":"video_out","to_node":"pass2","to_port":"video_in"}'
+
+# View graph
+curl http://localhost:8080/api/graph
+
+# Start all nodes
+curl -X POST http://localhost:8080/api/graph/start
+
+# Stop all nodes
+curl -X POST http://localhost:8080/api/graph/stop
+
+# Remove node
+curl -X DELETE http://localhost:8080/api/graph/nodes/pass1
+
+# Remove edge
+curl -X DELETE http://localhost:8080/api/graph/edges/pass1:video_out->pass2:video_in
+```
+
+### 6. Control node directly via WebSocket
+
+Connect to `ws://localhost:9100` with subprotocol `dmf-control`:
+
+```bash
+wscat -c ws://localhost:9100 -s dmf-control
+```
+
+Commands:
+```json
+{"cmd": "add_reader", "flow_id": "<uuid>", "port_id": "video_in"}
+{"cmd": "add_writer", "flow_id": "<uuid>", "port_id": "video_out", "flow_def": {<NMOS flow JSON>}}
+{"cmd": "remove_reader", "port_id": "video_in"}
+{"cmd": "remove_writer", "port_id": "video_out"}
+{"cmd": "configure", "params": {}}
+{"cmd": "shutdown"}
+```
+
+## Project structure
+
+```
+libs/dmf-node/     — Node skeleton library (interface, WS control, MXL lifecycle)
+libs/dmf-engine/   — Engine library (graph model, flow manager, process manager, REST API)
+nodes/passthrough/ — Passthrough node (1 MXL in → 1 MXL out, memcpy)
+engine/            — Engine binary
+tests/             — Unit tests
+```