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>

firmware/test/mocks/epd.h

@@ -0,0 +1,17 @@
+#pragma once
+#include "Arduino.h"
+
+// Call counters for assertions
+extern int g_epd_init_count;
+extern int g_epd_sleep_count;
+extern int g_epd_draw_image_count;
+extern int g_epd_fill_count;
+extern int g_epd_fill_last_color;
+extern int g_epd_draw_setup_count;
+
+inline void epd_init()  { g_epd_init_count++; }
+inline void epd_sleep() { g_epd_sleep_count++; }
+inline void epd_draw_image_from_file(File& f) { g_epd_draw_image_count++; }
+inline void epd_fill(int color) { g_epd_fill_count++; g_epd_fill_last_color = color; }
+inline void epd_draw_ap_screen(void*) {}
+inline void epd_draw_setup_screen(void*) { g_epd_draw_setup_count++; }