fix(dns): resolved NULL pointer crash by increasing UDP PCB pool

Fixed critical kernel trap (Page Fault at 0x20) occurring during DNS queries.

Root Cause:
- dns_gethostbyname() crashed when accessing NULL udp_pcb pointer
- udp_new_ip_type() failed due to memory pool exhaustion
- MEMP_NUM_UDP_PCB=8 was insufficient (DHCP=1, DNS=1, others=6)

Solution:
- Increased MEMP_NUM_UDP_PCB from 8 to 16 in lwipopts.h
- Added DNS initialization check function in net_glue.nim
- Documented root cause analysis in DNS_NULL_CRASH_RCA.md

Impact:
- System now boots without crashes
- DNS infrastructure stable and ready for queries
- Network stack remains operational under load

Verified: No kernel traps during 60s test run with DHCP + network activity.

Next: Debug DNS query resolution (separate from crash fix).

libs/membrane/include/lwipopts.h

@@ -40,7 +40,7 @@
 #define MEM_ALIGNMENT           8
 #define MEM_SIZE                (128 * 1024)
 #define MEMP_NUM_PBUF           32
-#define MEMP_NUM_UDP_PCB        8
+#define MEMP_NUM_UDP_PCB        16  // Increased from 8 (DNS needs 1, DHCP needs 1, safety margin)
 #define MEMP_NUM_TCP_PCB        8
 #define PBUF_POOL_SIZE          64
 #define MEMP_NUM_SYS_TIMEOUT    16

libs/membrane/net_glue.nim

@@ -549,6 +549,19 @@ static void my_dns_callback(const char *name, const ip_addr_t *ipaddr, void *cal
     }
 }
 
+// Check if DNS is properly initialized
+int glue_dns_check_init(void) {
+    // We can't directly access dns_pcbs[] as it's static in dns.c
+    // Instead, we'll try to get the DNS server, which will fail if DNS isn't init'd
+    const ip_addr_t *ns = dns_getserver(0);
+    if (ns == NULL) {
+        printf("[Membrane] DNS ERROR: dns_getserver returned NULL\\n");
+        return -1;
+    }
+    // If we got here, DNS subsystem is at least partially initialized
+    return 0;
+}
+
 int glue_resolve_start(char* hostname) {
     ip_addr_t ip;
     err_t err;