Export boot timestamp, update docs and examples to use it

src/sntp.h

@@ -1,10 +1,58 @@
+// Time / SNTP functions
+
 #pragma once
 
 #include "net.h"
 
+// Return milliseconds since boot (uptime). Never goes backwards.
+// On RTOS targets (FreeRTOS, ThreadX, Zephyr, etc.) uses the RTOS tick count.
+// On STM32/Cube uses HAL_GetTick(). On Arduino uses millis().
+// On Linux/macOS uses CLOCK_MONOTONIC_RAW or CLOCK_MONOTONIC; falls back to
+// CLOCK_REALTIME if neither is available.
+uint64_t mg_millis(void);
+
+// Boot timestamp in ms since Epoch. Updated by successful SNTP request
+extern uint64_t mg_boot_timestamp_ms;
+
+// Return milliseconds since Epoch: mg_millis() + mg_boot_timestamp_ms.
+// Until a successful SNTP request completes, this is identical to mg_millis().
+uint64_t mg_now(void);
+
+// Return true if the periodic timer has fired; advance `expiration` by one
+// `period`. Handles wrap-around. Usage example:
+//
+// ```c
+// uint64_t timer = 0;
+// for (;;) {
+//   if (mg_timer_expired(&timer, 1000, mg_millis())) {
+//     MG_INFO(("Hi"));  // Print a message every second
+//   }
+//   mg_mgr_poll(&mgr, 10);
+// }
+// ```
+bool mg_timer_expired(uint64_t *expiration, uint64_t period, uint64_t now);
+
+// Connect to an SNTP server and send a time request.
+// `url` defaults to "udp://time.google.com:123" when NULL.
+// On success the internal boot timestamp is updated so that mg_now() returns
+// the correct wall-clock time, and MG_EV_SNTP_TIME is fired on `fn` (if not
+// NULL) with a pointer to the int64_t epoch in milliseconds.
+// Pass fn == NULL and fn_data == NULL for a fire-and-forget sync that only
+// updates mg_boot_timestamp_ms. Typical polling usage:
+//
+// ```c
+// uint64_t timer = 0;
+// for (;;) {
+//   uint64_t period = mg_boot_timestamp_ms == 0 ? 1000 : 3600 * 1000;
+//   if (mg_timer_expired(&timer, period, mg_millis())) {
+//     mg_sntp_connect(&mgr, NULL, NULL, NULL);
+//   }
+//   mg_mgr_poll(&mgr, 1000);
+// }
+// ```
 struct mg_connection *mg_sntp_connect(struct mg_mgr *mgr, const char *url,
                                       mg_event_handler_t fn, void *fn_data);
+
+// Private API, do not expose
 void mg_sntp_request(struct mg_connection *c);
 int64_t mg_sntp_parse(const unsigned char *buf, size_t len);
-
-uint64_t mg_now(void);     // Return milliseconds since Epoch