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/LittleFS.h

@@ -0,0 +1,39 @@
+#pragma once
+#include "Arduino.h"
+#include <map>
+#include <vector>
+
+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; }
+};
+
+struct LittleFSClass {
+    std::map<std::string, std::string> files;
+
+    bool begin(bool) { return true; }
+
+    File open(const char* path, const char* mode, bool create = false) {
+        File f;
+        f._valid = true;
+        f._write = (mode[0] == 'w');
+        f._buf = &files[path];
+        if (f._write) f._buf->clear();
+        f._pos = 0;
+        return f;
+    }
+} LittleFS;