feat: NDI in node receives and writes audio flow

- FlowDef.hpp: add make_audio_flow_def (NMOS IS-04 audio/Lnn flow)
- NDIReceiver: replace capture_v210 with capture() — single recv call
  dispatches to FrameKind::{Video,Audio,None}; AudioInfo struct carries
  sample_rate/channels/samples/channel_stride
- ndiin: optional audio writer when graph wires audio_flow_id; single
  loop with 5ms NDI poll; audio grains written immediately on arrival;
  video grains written when mxlGetCurrentIndex reaches video_index
- build_graph: rename ndiin video port flow_id → video_flow_id; add
  audio_flow_id edge (no sink yet — wired to ndiin only)

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

studio-manager/main.cpp

@@ -39,16 +39,23 @@ static std::string gen_uuid() {
 static dmf::FlowGraph build_graph() {
     dmf::FlowGraph g;
     const std::string video_flow = gen_uuid();
+    const std::string audio_flow = gen_uuid();
     g.nodes = {
         { "ndiin",    "ndiin",    {} },
         { "fakesink", "fakesink", {} },
         { "ndiout",   "ndiout",   {} },
     };
+    const nlohmann::json video_fmt = {
+        {"kind","video"}, {"width",1920}, {"height",1080}, {"fps_num",25}, {"fps_den",1}
+    };
+    const nlohmann::json audio_fmt = {
+        {"kind","audio"}, {"sample_rate",48000}, {"channels",2}, {"bit_depth",32}
+    };
     g.edges = {
-        { video_flow, "ndiin", "flow_id", "fakesink", "flow_id",
-          { {"kind","video"}, {"width",1920}, {"height",1080}, {"fps_num",25}, {"fps_den",1} } },
-        { video_flow, "ndiin", "flow_id", "ndiout",   "flow_id",
-          { {"kind","video"}, {"width",1920}, {"height",1080}, {"fps_num",25}, {"fps_den",1} } },
+        { video_flow, "ndiin", "video_flow_id", "fakesink", "flow_id", video_fmt },
+        { video_flow, "ndiin", "video_flow_id", "ndiout",   "flow_id", video_fmt },
+        // audio_flow_id wired to ndiin only — no sink node yet, readers added later
+        { audio_flow, "ndiin", "audio_flow_id", "",         "",        audio_fmt },
     };
     return g;
 }