fix: replace real-time unsafe malloc with stack buffers in process_callback

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

engine.c

@@ -1,5 +1,7 @@
 #include "engine.h"
 #include "carla.h"
+
+#define MAX_NFRAMES 8192
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -125,13 +127,11 @@ static int process_callback(jack_nframes_t nframes, void *arg) {
     for (int ch = 0; ch < MAX_CHANNELS; ch++) {
         memset(audio_out[ch], 0, sizeof(jack_default_audio_sample_t) * nframes);
         
-        // Create temporary buffer for rack processing
-        float *rack_in = malloc(nframes * sizeof(float));
-        float *rack_out = malloc(nframes * sizeof(float));
-        
-        if (!rack_in || !rack_out) {
-            free(rack_in);
-            free(rack_out);
+        // Use stack-allocated buffers (max MAX_NFRAMES samples)
+        float rack_in[MAX_NFRAMES];
+        float rack_out[MAX_NFRAMES];
+        if (nframes > MAX_NFRAMES) {
+            // Should never happen with JACK, but guard
             continue;
         }
         
@@ -164,8 +164,6 @@ static int process_callback(jack_nframes_t nframes, void *arg) {
         // Copy to output
         memcpy(audio_out[ch], rack_out, nframes * sizeof(jack_default_audio_sample_t));
         
-        free(rack_in);
-        free(rack_out);
     }
     
     return 0;