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>

platformio.ini

@@ -24,3 +24,10 @@ build_flags = -DENV_TEST_DISPLAY
 build_src_filter = +<epd.cpp> +<test_display.cpp>
 lib_deps =
     ricmoo/QRCode@^0.0.1
+
+[env:native-test]
+platform = native
+lib_deps =
+    throwtheswitch/Unity@^2.6
+build_flags = -DUNIT_TEST -std=c++17 -iquote test/mocks -iquote test -Itest/mocks -Itest
+test_build_src = no