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>

test/mocks/driver/gpio.h

@@ -0,0 +1,11 @@
+#pragma once
+// Native-test stubs for gpio_hold_* — operation.h calls
+// gpio_deep_sleep_hold_en() before esp_deep_sleep_start() to latch
+// PIN_PWR LOW through deep sleep. epd_driver.cpp (not built natively)
+// also uses gpio_hold_en / gpio_hold_dis. Stubs let tests link.
+#include "esp_sleep.h"  // for gpio_num_t
+
+inline void gpio_hold_en(gpio_num_t) {}
+inline void gpio_hold_dis(gpio_num_t) {}
+inline void gpio_deep_sleep_hold_en() {}
+inline void gpio_deep_sleep_hold_dis() {}