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/Signal.hpp

@@ -0,0 +1,20 @@
+#pragma once
+#include <atomic>
+#include <csignal>
+
+namespace dmf {
+
+inline std::atomic<bool> g_running{true};
+
+namespace detail {
+    inline void on_signal(int) noexcept {
+        g_running.store(false, std::memory_order_relaxed);
+    }
+}
+
+inline void install_signal_handlers() {
+    std::signal(SIGTERM, detail::on_signal);
+    std::signal(SIGINT,  detail::on_signal);
+}
+
+} // namespace dmf