fix: add required grouphint tag to make_audio_flow_def

MXL FlowParser requires urn:x-nmos:tag:grouphint/v1.0 in the tags
object of every flow definition. make_audio_flow_def was missing tags
entirely, causing FlowParser.cpp:192 "Invalid group hint tag".

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

shared/FlowDef.hpp

@@ -65,6 +65,9 @@ inline std::string make_audio_flow_def(
         {"sample_rate", {{"numerator", sample_rate},    {"denominator", 1}}},
         {"channels",    ch_arr},
         {"bit_depth",   bit_depth},
+        {"tags", {
+            {"urn:x-nmos:tag:grouphint/v1.0", json::array({label + ":Audio"})}
+        }},
     }.dump();
 }