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/panels/waveshare13e6/v1/epd_driver.cpp

@@ -24,6 +24,7 @@
 #include <qrcode.h>
 #include <esp_task_wdt.h>
 #include <esp_heap_caps.h>
+#include <driver/gpio.h>
 
 static constexpr uint16_t W              = 1200;
 static constexpr uint16_t H              = 1600;
@@ -103,6 +104,12 @@ static void panel_reset() {
 }
 
 void epd_setup_pins() {
+    // Release the PIN_PWR hold latched from the previous deep sleep so we
+    // can drive it again. No-op on cold boot (nothing was held). Paired
+    // with gpio_hold_en() in epd_sleep() and gpio_deep_sleep_hold_en() in
+    // operation.h.
+    gpio_hold_dis((gpio_num_t)PIN_PWR);
+
     pinMode(PIN_PWR,  OUTPUT);
     pinMode(PIN_RST,  OUTPUT);
     pinMode(PIN_DC,   OUTPUT);
@@ -191,6 +198,16 @@ void epd_sleep() {
     SPI.transfer(0xA5);                          // sentinel
     cs_both(HIGH);
     s_initialized = false;
+
+    // Cut the panel power rail. The Waveshare board exposes PIN_PWR
+    // specifically for battery operation — the e-ink image persists
+    // without VDD (particles are bistable), and dropping the rail kills
+    // the boost converter's quiescent draw (~50–500 µA depending on the
+    // load on the rail). Latch LOW so it survives deep sleep; paired with
+    // gpio_deep_sleep_hold_en() in operation.h just before the chip
+    // enters sleep.
+    digitalWrite(PIN_PWR, LOW);
+    gpio_hold_en((gpio_num_t)PIN_PWR);
 }
 
 // ── Draw helpers ───────────────────────────────────────────────────────────────