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>

nodes/fakesink/main.cpp

@@ -25,7 +25,7 @@ class FakeSinkNode : public dmf::NodeBase {
         mxlFlowReader reader{};
         mxlStatus st = mxlCreateFlowReader(instance(), flow_id.c_str(), nullptr, &reader);
         if (st != MXL_STATUS_OK) {
-            log("mxlCreateFlowReader failed (status=%d)", st);
+            log("mxlCreateFlowReader failed (%s)", dmf::mxl_status_str(st));
             return;
         }
 
@@ -60,7 +60,7 @@ class FakeSinkNode : public dmf::NodeBase {
                 index = ri.headIndex;
 
             } else {
-                log("unexpected status=%d on index=%llu", st, index);
+                log("unexpected status (%s) on index=%llu", dmf::mxl_status_str(st), index);
                 break;
             }