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/FS.h

@@ -0,0 +1,30 @@
+#pragma once
+// Minimal FS.h stub for native unit tests
+// Provides the fs::File type that Arduino's FS library normally defines.
+
+#include "Arduino.h"
+#include <string>
+#include <cstdint>
+
+namespace fs {
+
+struct File {
+    std::string* _buf = nullptr;
+    bool _valid = false;
+    bool _write = false;
+    size_t _pos = 0;
+
+    explicit operator bool() const { return _valid; }
+    void close() { _valid = false; }
+    size_t write(const uint8_t* data, size_t len) {
+        if (_buf && _write) { _buf->append((const char*)data, len); return len; }
+        return 0;
+    }
+    int read() {
+        if (_buf && _pos < _buf->size()) return (uint8_t)(*_buf)[_pos++];
+        return -1;
+    }
+    size_t size() { return _buf ? _buf->size() : 0; }
+};
+
+} // namespace fs