Phase 4: TollGate client detection + auto-payment

- New tollgate_client.c/h: detect upstream TollGate (kind=10021),
  auto-pay via nucula wallet, session monitoring with 20% renewal
- State machine: IDLE→DETECTING→NEEDS_PAY→PAYING→PAID→RENEWING
- Blocking: upstream payment before local services start
- Synchronous wallet init (was async task)
- Client config: enabled, steps_to_buy, renewal_threshold_pct
- Updated PLAN.md with Phases 4-7 (client, payout, bytes, CVM)
- Updated CHECKLIST.md with all new phase items
- 30 new unit tests (all passing), 116 total

2 files changed, 18 insertions, 0 deletions

diff --git a/tests/unit/stubs/esp_err.h b/tests/unit/stubs/esp_err.h
index 9bedb72..2a8e216 100644
--- a/tests/unit/stubs/esp_err.h
+++ b/tests/unit/stubs/esp_err.h
@@ -12,6 +12,7 @@ typedef int esp_err_t;
 #define ESP_ERR_INVALID_ARG  0x102
 #define ESP_ERR_NO_MEM       0x101
 #define ESP_ERR_NOT_FOUND    0x104
+#define ESP_ERR_INVALID_STATE 0x103
 
 static inline const char *esp_err_to_name(esp_err_t err) { (void)err; return "ESP_OK"; }
 
diff --git a/tests/unit/stubs/nucula_wallet.h b/tests/unit/stubs/nucula_wallet.h
new file mode 100644
index 0000000..260ec35
--- /dev/null
+++ b/tests/unit/stubs/nucula_wallet.h
@@ -0,0 +1,17 @@
+#ifndef STUBS_NUCULA_WALLET_H
+#define STUBS_NUCULA_WALLET_H
+
+#include "esp_err.h"
+#include <stdint.h>
+#include <stddef.h>
+
+esp_err_t nucula_wallet_init(const char *mint_url);
+esp_err_t nucula_wallet_receive(const char *token_str);
+esp_err_t nucula_wallet_send(uint64_t amount_sat, char *token_out, size_t token_out_size);
+uint64_t nucula_wallet_balance(void);
+int nucula_wallet_proof_count(void);
+char *nucula_wallet_proofs_json(void);
+esp_err_t nucula_wallet_swap_all(void);
+void nucula_wallet_print_status(void);
+
+#endif