Eliminate VLA for stack safety and portability

Variable-length arrays (VLAs) were banned from the Linux kernel starting
with 4.20 due to stack overflow risks and unpredictable stack usage.
This replaces the VLA in picosynth_process() with a fixed-size scratch
array bounded by PICOSYNTH_MAX_NODES (default 32).

Stack usage is now predictable: 128 bytes per voice processing call.

Author: jserv

include/picosynth.h

@@ -44,6 +44,19 @@
 #error "PICOSYNTH_BLOCK_SIZE must be <= 255 (uint8_t block_counter)"
 #endif
 
+/* Maximum nodes per voice. Fixed-size scratch array avoids VLAs
+ * (banned from Linux kernel due to stack overflow risk).
+ * Default 32 is sufficient for complex patches; increase if needed.
+ * picosynth_create() returns NULL if nodes exceeds this limit.
+ */
+#ifndef PICOSYNTH_MAX_NODES
+#define PICOSYNTH_MAX_NODES 32
+#endif
+
+#if PICOSYNTH_MAX_NODES > 255
+#error "PICOSYNTH_MAX_NODES must be <= 255 (uint8_t n_nodes)"
+#endif
+
 /**
  * Q15 fixed-point: signed 16-bit, 15 fractional bits.
  * Range: [-1.0, +1.0) as [-32768, +32767].

src/picosynth.c

@@ -153,6 +153,9 @@ static void voice_update_usage_mask(picosynth_voice_t *v)
 
 picosynth_t *picosynth_create(uint8_t voices, uint8_t nodes)
 {
+    if (nodes > PICOSYNTH_MAX_NODES)
+        return NULL;
+
     picosynth_t *s = calloc(1, sizeof(picosynth_t));
     if (!s)
         return NULL;
@@ -475,7 +478,7 @@ q15_t picosynth_process(picosynth_t *s)
 
         picosynth_voice_t *v = &s->voices[vi];
         picosynth_node_t *nodes = v->nodes;
-        int32_t tmp[v->n_nodes];
+        int32_t tmp[PICOSYNTH_MAX_NODES];
 
         /* Two-pass processing per voice:
          * 1. Compute outputs from current state of all nodes.