Test BLE-coex wedge hypothesis: C6 slave rebuilt with Bluetooth off

The wedge clock is immune to every network-layer change (~300s
regardless of bus width, clock, power-save, DHCP). A slave-side
periodic starver remains; the C6's BLE controller (BT_LE_SLEEP_ENABLE=y
in stock slave, shares the radio/RF arbiter with Wi-Fi) is the prime
suspect. DeskLock uses no Bluetooth, so the stock slave carries a
coexistence scheduler we can delete outright. Custom slave 2.12.12
(CONFIG_BT_ENABLED=n, ESP_HOSTED_CP_BT=n), OTA'd over SDIO. If the
metronome stops, BLE coexistence was starving the SDIO peripheral.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

firmware/main/c6_ota.c

@@ -44,8 +44,8 @@ static void c6_ota_task(void *arg)
     vTaskDelay(pdMS_TO_TICKS(8000)); /* let the hosted transport settle */
     esp_hosted_coprocessor_fwver_t v = { 0 };
     if (esp_hosted_get_coprocessor_fwversion(&v) == ESP_OK &&
-        v.major1 >= 2) {
-        ESP_LOGI(TAG, "C6 fw already 2.x — no OTA needed");
+        v.major1 == 2 && v.minor1 == 12 && v.patch1 == 12) {
+        ESP_LOGI(TAG, "C6 fw already 2.12.12-btfree — no OTA needed");
         vTaskDelete(NULL);
         return;
     }