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>

studio-manager/main.cpp

@@ -10,6 +10,7 @@
 #include <fcntl.h>
 #include <sys/wait.h>
 #include <unistd.h>
+#include <mxl/mxl.h>
 #include <mxl/time.h>
 #include "Signal.hpp"
 #include "FlowGraph.hpp"
@@ -152,6 +153,16 @@ int main(int argc, char* argv[]) {
     }
     fprintf(stderr, "[studio-manager] domain: %s\n", domain.c_str());
 
+    // Clean up stale flow directories left by any previous crashed run
+    {
+        mxlInstance gc = mxlCreateInstance(domain.c_str(), nullptr);
+        if (gc) {
+            mxlGarbageCollectFlows(gc);
+            mxlDestroyInstance(gc);
+            fprintf(stderr, "[studio-manager] garbage collected stale flows\n");
+        }
+    }
+
     // --- Build and launch the pipeline graph ---
     const dmf::FlowGraph graph = build_graph();