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>

shared/FlowDef.hpp

@@ -36,4 +36,36 @@ inline std::string make_video_flow_def(
     }.dump();
 }
 
+// Generates a minimal but valid NMOS IS-04 flow definition JSON string
+// for a raw PCM audio flow. grain_rate_num/den sets the grain delivery rate
+// (default 25/1 = one grain per video frame). MXL computes grain size as
+// sample_rate / grain_rate samples per grain.
+inline std::string make_audio_flow_def(
+    const std::string& flow_id,
+    const std::string& label,
+    int sample_rate,
+    int channels,
+    int bit_depth       = 32,
+    int grain_rate_num  = 25,
+    int grain_rate_den  = 1)
+{
+    using json = nlohmann::json;
+    static const char* ch_labels[] = {"L","R","C","LFE","Ls","Rs","Lss","Rss"};
+    json ch_arr = json::array();
+    for (int i = 0; i < channels; ++i)
+        ch_arr.push_back({{"label", i < 8 ? ch_labels[i] : ("Ch" + std::to_string(i + 1))}});
+    return json{
+        {"id",          flow_id},
+        {"format",      "urn:x-nmos:format:audio"},
+        {"label",       label},
+        {"description", label + " MXL Audio Flow"},
+        {"media_type",  "audio/L" + std::to_string(bit_depth)},
+        {"parents",     json::array()},
+        {"grain_rate",  {{"numerator", grain_rate_num}, {"denominator", grain_rate_den}}},
+        {"sample_rate", {{"numerator", sample_rate},    {"denominator", 1}}},
+        {"channels",    ch_arr},
+        {"bit_depth",   bit_depth},
+    }.dump();
+}
+
 } // namespace dmf