feat: add Carla plugin host stubs and integration plan

client/src/carla_host.c

@@ -0,0 +1,38 @@
+#include <stddef.h>
+#include "carla_host.h"
+
+int carla_load(const char *binary, const char *plugin_id, int *out_id)
+{
+    (void)plugin_id;
+    (void)out_id;
+    if (binary == NULL) return -1;
+    // stub: always fails (will be replaced by real Carla later)
+    return -1;
+}
+
+int carla_unload(int id)
+{
+    (void)id;
+    return -1; // stub: always fails
+}
+
+int carla_connect(int id, const char *port_name, const char *looper_port)
+{
+    (void)id;
+    (void)port_name;
+    (void)looper_port;
+    return -1; // stub: always fails
+}
+
+int carla_disconnect(const char *from, const char *to)
+{
+    (void)from;
+    (void)to;
+    return 0; // stub: disconnect always succeeds (does nothing)
+}
+
+void carla_set_bypass(int id, bool bypass)
+{
+    (void)id;
+    (void)bypass;
+}