feat(operation): send X-Boot-Reason so power-cycle is a force-resync

Distinguish a cold-boot poll (UNDEFINED wakeup cause = power-on, hard
reset, plug-cycle) from a normal timer wake. Encoded as the
X-Boot-Reason request header; server uses it to deliberately bypass
the schedule and rotate. Matches how users actually use the device:
unplug-and-replug as a manual refresh.

Tests: two new native cases asserting the header is "cold" on
UNDEFINED wakeup and "timer" on TIMER wakeup. esp_sleep mock now
exposes a settable wakeup_cause global.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/operation.h

@@ -127,6 +127,15 @@ void normal_operation_impl(const String& mac, HTTP& http, const String& url, Pre
         http.addHeader("X-Current-Image-Id", String(currentImgId));
     }
 
+    // Tell the server how we got here. The server uses this to honor a
+    // power-cycle as a deliberate "force resync" — a poll that arrives with
+    // X-Boot-Reason: cold gets a fresh rotation even outside configured wake
+    // times, so unplugging and replugging the frame works as a manual refresh.
+    // Timer wakes (the normal case) keep their schedule-gated semantics.
+    const esp_sleep_wakeup_cause_t cause = esp_sleep_get_wakeup_cause();
+    http.addHeader("X-Boot-Reason",
+                   cause == ESP_SLEEP_WAKEUP_TIMER ? "timer" : "cold");
+
     const char* collectHeaders[] = { "X-Interval-Ms", "X-Image-Id", "X-Image-Sha256" };
     http.collectHeaders(collectHeaders, 3);
     int code = http.GET();