feat: garbage collection on startup, readable status strings, audio/float32

studio-manager: call mxlGarbageCollectFlows before launching nodes to
clean up stale flow directories left by previous crashed runs.

shared/NodeBase.hpp: add mxl_status_str(mxlStatus) — converts error
codes to readable names (e.g. MXL_ERR_OUT_OF_RANGE_TOO_LATE). All
nodes now log these names instead of raw integers.

shared/FlowDef.hpp: use "audio/float32" as media_type for 32-bit audio
flows, matching the MXL SDK examples. audio/L{n} kept for other depths.

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

shared/FlowDef.hpp

@@ -59,7 +59,7 @@ inline std::string make_audio_flow_def(
         {"format",      "urn:x-nmos:format:audio"},
         {"label",       label},
         {"description", label + " MXL Audio Flow"},
-        {"media_type",  "audio/L" + std::to_string(bit_depth)},
+        {"media_type",  bit_depth == 32 ? "audio/float32" : "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}}},