fix: add channel_count to make_audio_flow_def

MXL reads channel_count (plain integer) to determine grain buffer
geometry, defaulting to 1 if absent. The NMOS IS-04 channels array
is ignored by MXL. Without channel_count, stereo audio grains were
half the required size.

Also: MXL only accepts bit_depth 32 or 64 for audio flows.

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

shared/FlowDef.hpp

@@ -63,8 +63,9 @@ inline std::string make_audio_flow_def(
         {"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},
+        {"channels",       ch_arr},   // NMOS IS-04 metadata (ignored by MXL)
+        {"channel_count",  channels},  // MXL uses this for grain buffer geometry
+        {"bit_depth",      bit_depth}, // must be 32 or 64
         {"tags", {
             {"urn:x-nmos:tag:grouphint/v1.0", json::array({label + ":Audio"})}
         }},