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/SPI.h

@@ -0,0 +1,8 @@
+#pragma once
+struct SPISettings { SPISettings(uint32_t, uint8_t, uint8_t) {} };
+struct SPIClass {
+    void begin(int,int,int,int) {}
+    void beginTransaction(SPISettings) {}
+} SPI;
+#define MSBFIRST 1
+#define SPI_MODE0 0