Initial commit: testpattern → fakesink pipeline over MXL shared memory

- NodeBase, Signal, FlowDef, V210 shared headers
- testpattern node: SMPTE 75% color bars writer at 25fps
- fakesink node: non-blocking MXL reader with per-second stats
- studio-manager: FlowGraph data model, graph-driven fork/exec launcher
- mxl pinned as submodule at 0ae1dc5

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shared/FlowDef.hpp

@@ -0,0 +1,39 @@
+#pragma once
+#include <string>
+#include <nlohmann/json.hpp>
+
+namespace dmf {
+
+// Generates a minimal but valid NMOS IS-04 flow definition JSON string
+// for a video/v210 flow. MXL uses this to set up the ring buffer geometry.
+inline std::string make_video_flow_def(
+    const std::string& flow_id,
+    const std::string& label,
+    int width, int height,
+    int fps_num, int fps_den = 1)
+{
+    using json = nlohmann::json;
+    return json{
+        {"id",             flow_id},
+        {"format",         "urn:x-nmos:format:video"},
+        {"label",          label},
+        {"description",    label + " MXL Video Flow"},
+        {"media_type",     "video/v210"},
+        {"parents",        json::array()},
+        {"grain_rate",     {{"numerator", fps_num}, {"denominator", fps_den}}},
+        {"frame_width",    width},
+        {"frame_height",   height},
+        {"interlace_mode", "progressive"},
+        {"colorspace",     "BT709"},
+        {"tags", {
+            {"urn:x-nmos:tag:grouphint/v1.0", json::array({label + ":Video"})}
+        }},
+        {"components", json::array({
+            {{"name","Y"},  {"width",width},   {"height",height}, {"bit_depth",10}},
+            {{"name","Cb"}, {"width",width/2}, {"height",height}, {"bit_depth",10}},
+            {{"name","Cr"}, {"width",width/2}, {"height",height}, {"bit_depth",10}}
+        })}
+    }.dump();
+}
+
+} // namespace dmf