feat(13e6): cut panel power rail in deep sleep via PIN_PWR + GPIO hold

The Waveshare board exposes PIN_PWR (GPIO 1) specifically so battery
designs can gate the panel rail between refreshes. Before this commit
PIN_PWR was driven HIGH at boot and never released, so the panel's
boost converter kept its quiescent draw (50–500 µA) through every
deep sleep. The e-ink particles are bistable so the displayed image
persists without VDD; dropping the rail is a free win.

Three pieces:
  • epd_sleep() drives PIN_PWR LOW after issuing the panel-internal
    DEEP_SLEEP command, then gpio_hold_en() latches the level so it
    survives the chip's RTC transition.
  • normal_operation_impl() calls gpio_deep_sleep_hold_en() just
    before esp_deep_sleep_start() so the per-pin hold extends through
    the deep sleep period itself (without this the holds release on
    the transition and the rail comes back up).
  • epd_setup_pins() calls gpio_hold_dis() at the very top to free
    PIN_PWR on wake before re-driving it HIGH; no-op on cold boot.

Tests: 47/47 pass. Added test/mocks/driver/gpio.h with no-op stubs so
the native test build links cleanly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/operation.h

@@ -9,9 +9,11 @@
 // The test build adds test/mocks to the include path via -iquote.
 #include "epd_mock.h"
 #include "esp_sleep.h"
+#include "driver/gpio.h"
 #else
 #include "epd.h"
 #include <esp_sleep.h>
+#include <driver/gpio.h>
 #include <mbedtls/sha256.h>
 #endif
 
@@ -354,6 +356,12 @@ void normal_operation_impl(const String& mac, HTTP& http, const String& url, Pre
     // false → normal_operation_impl runs → the next poll fetches a fresh
     // image, which doubles as a "force refresh" gesture.
     esp_sleep_enable_ext0_wakeup((gpio_num_t)PIN_BTN_RESET, 0);
+    // Latch any gpio_hold_en pins through the deep sleep period.
+    // epd_sleep() cuts PIN_PWR LOW + holds it; without this call the
+    // hold releases on the RTC transition and the panel rail comes back
+    // up, losing the saving. Released per-pin in epd_setup_pins() on
+    // wake via gpio_hold_dis().
+    gpio_deep_sleep_hold_en();
     esp_deep_sleep_start();
 }