fix: harden firmware NVS persistence, WDT, and 304 epd_sleep

Three bugs fixed:
- NVS img_id now written before epd_init/draw; new draw_needed flag in NVS
  survives power-loss mid-refresh so next boot re-draws from LittleFS instead
  of showing stale content
- epd_sleep() now only called when display was initialized this cycle,
  preventing a 60 s wait_busy() timeout on every 304 poll
- esp_task_wdt_reset() added to wait_busy() loop so the ~20 s 6-color
  refresh no longer triggers the task watchdog

Also extracts normal_operation into operation.h template and adds
a native PlatformIO test suite (16 tests) covering the full response matrix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/mocks/esp_sleep.h

@@ -0,0 +1,8 @@
+#pragma once
+#include <cstdint>
+
+extern uint64_t g_sleep_us;
+extern bool     g_deep_sleep_started;
+
+inline void esp_sleep_enable_timer_wakeup(uint64_t us) { g_sleep_us = us; }
+inline void esp_deep_sleep_start() { g_deep_sleep_started = true; }