feat: per-client NAT filtering via LWIP_HOOK_IP4_CANFORWARD

- Add lwip_tollgate_hooks.h defining LWIP_HOOK_IP4_CANFORWARD macro
- Inject hook into lwIP build via CMakeLists.txt ESP_IDF_LWIP_HOOK_FILENAME
- Filter forwarded packets by source IP against firewall allowed list
- Only filter packets from AP subnet (10.192.45.0/24), allow all others
- Fix byte order bug: use network byte order for firewall_is_client_allowed
- NAT always enabled, removed global NAT toggle functions
- Remove spent-secret tracking from session.c (mint is authority)
- Remove unused get_ap_netif() function
- Reduce API server stack from 32KB to 16KB (fixes ESP_ERR_HTTPD_TASK)
- Add esp_random.h stub for unit tests
- All 186 unit tests passing
- Verified on hardware: block->pay->allow->revoke->block E2E works

1 files changed, 10 insertions, 0 deletions

diff --git a/main/lwip_tollgate_hooks.h b/main/lwip_tollgate_hooks.h
new file mode 100644
index 0000000..76017be
--- /dev/null
+++ b/main/lwip_tollgate_hooks.h
@@ -0,0 +1,10 @@
+#ifndef LWIP_TOLLGATE_HOOKS_H
+#define LWIP_TOLLGATE_HOOKS_H
+
+#include "lwip/pbuf.h"
+
+int tollgate_ip4_canforward_filter(struct pbuf *p, u32_t dest_addr_hostorder);
+
+#define LWIP_HOOK_IP4_CANFORWARD(p, addr) tollgate_ip4_canforward_filter(p, addr)
+
+#endif