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>

test/mocks/Arduino.h

@@ -0,0 +1,98 @@
+#pragma once
+#include <string>
+#include <cstring>
+#include <cstdint>
+#include <cstdlib>
+#include <algorithm>
+#include <sstream>
+#include <cctype>
+
+// Minimal String class that mimics Arduino's String for native tests
+struct String {
+    std::string _s;
+
+    String() {}
+    String(const char* s) : _s(s ? s : "") {}
+    String(const std::string& s) : _s(s) {}
+    String(int v) { _s = std::to_string(v); }
+    String(unsigned long v) { _s = std::to_string(v); }
+    String(long long v) { _s = std::to_string(v); }
+    String(unsigned long long v) { _s = std::to_string(v); }
+
+    const char* c_str() const { return _s.c_str(); }
+    size_t length() const { return _s.size(); }
+    bool isEmpty() const { return _s.empty(); }
+    bool empty() const { return _s.empty(); }
+
+    int toInt() const { return _s.empty() ? 0 : std::stoi(_s); }
+
+    String substring(size_t from) const { return String(_s.substr(from)); }
+    String substring(size_t from, size_t to) const { return String(_s.substr(from, to - from)); }
+
+    void replace(const char* from, const char* to_str) {
+        std::string result;
+        std::string f(from), t(to_str);
+        size_t pos = 0, found;
+        while ((found = _s.find(f, pos)) != std::string::npos) {
+            result += _s.substr(pos, found - pos);
+            result += t;
+            pos = found + f.size();
+        }
+        result += _s.substr(pos);
+        _s = result;
+    }
+
+    void toUpperCase() {
+        for (char& c : _s) c = (char)toupper((unsigned char)c);
+    }
+
+    bool operator==(const String& o) const { return _s == o._s; }
+    bool operator==(const char* o) const { return _s == o; }
+    bool operator!=(const String& o) const { return _s != o._s; }
+    bool operator!=(const char* o) const { return _s != o; }
+
+    String operator+(const String& o) const { return String(_s + o._s); }
+    String operator+(const char* o) const { return String(_s + o); }
+    String& operator+=(const String& o) { _s += o._s; return *this; }
+    String& operator+=(const char* o) { _s += o; return *this; }
+
+    // Allow use as map key
+    operator std::string() const { return _s; }
+
+    // toString() for IP-like objects that have it
+    String toString() const { return *this; }
+};
+
+inline String operator+(const char* a, const String& b) { return String(std::string(a) + b._s); }
+inline String operator+(const std::string& a, const String& b) { return String(a + b._s); }
+
+// Controllable millis and digitalRead for timeout / button tests
+extern uint32_t g_millis_value;
+extern int      g_digital_read_value;
+
+#ifndef LOW
+#define LOW  0
+#define HIGH 1
+#endif
+
+inline unsigned long millis() { return g_millis_value += 10; }
+inline void delay(unsigned long) {}
+inline void pinMode(int, int) {}
+inline int digitalRead(int) { return g_digital_read_value; }
+
+// Color constants (from config.h)
+#define COLOR_YELLOW 0x2
+#define COLOR_RED    0x3
+
+// Serial mock
+struct SerialMock {
+    void begin(int) {}
+    void println(const String&) {}
+    void println(const char*) {}
+    void println(int) {}
+    void print(const String&) {}
+    void print(const char*) {}
+    void flush() {}
+} Serial;
+
+// strtoull is available from <cstdlib> on native