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/Preferences.h

@@ -0,0 +1,35 @@
+#pragma once
+#include "Arduino.h"
+#include <map>
+
+// Shared sequence counter — incremented by each instrumented mock call
+extern int g_call_seq;
+extern int g_prefs_putint_seq;  // sequence position of last putInt call
+
+struct Preferences {
+    std::map<std::string, int32_t>  ints;
+    std::map<std::string, std::string> strings;
+    bool _open = false;
+
+    void begin(const char*, bool) { _open = true; }
+    void end() { _open = false; }
+
+    int32_t getInt(const char* key, int32_t def = 0) {
+        auto it = ints.find(key);
+        return it != ints.end() ? it->second : def;
+    }
+    void putInt(const char* key, int32_t val) {
+        ints[key] = val;
+        // Record the sequence of the FIRST putInt call (ordering test uses this
+        // to verify NVS is written before epd_draw_image_from_file).
+        if (g_prefs_putint_seq < 0) g_prefs_putint_seq = g_call_seq;
+        g_call_seq++;
+    }
+
+    String getString(const char* key, const char* def = "") {
+        auto it = strings.find(key);
+        return it != strings.end() ? String(it->second) : String(def);
+    }
+    void putString(const char* key, const String& val) { strings[key] = val._s; }
+    void clear() { ints.clear(); strings.clear(); }
+};