fix: post-review issues + close M2/M3/M4 from internal backlog (v1.9.9)

From Sonnet pre-release code review (3 real issues):
- F1 (high): factory reset atomicity — save cleared creds BEFORE zeroing reset
  counter, so a power loss between commits still results in WiFiManager AP boot
  instead of inconsistent "counter=0 + stale creds" state
- F2: isValidSSID now allows single-char SSIDs (per 802.11 spec)
- F3: ntp_interval changes now trigger needsRestart (NTPClient constructed once
  in setup() with this value, doesn't pick up runtime changes)

From internal backlog:
- M2: setupWiFi Try-2 now supports open networks (no password) — fixed the
  &&-condition that required both ssid and password
- M3: removed dead NTP_WAITING/NTP_SUCCESS/NTP_FAILED enum values
- M4: randomSeed() with ESP.getChipId() ^ micros() — dissolve pattern varies

Tested on hardware: 72/73 tests pass (single failure is expected — ntp_interval
fuzz cases now trigger restart due to F3, second case lands in reboot window).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/weather_clock/weather_clock.ino

@@ -133,16 +133,22 @@ void ICACHE_FLASH_ATTR checkFactoryReset() {
 
   if (rc.count >= RESET_COUNTER_TRIPS) {
     Serial.println("!!! FACTORY RESET triggered !!!");
+
+    // Atomicity: clear credentials FIRST (saveConfig commits), then zero counter.
+    // If power fails between the two commits, the device boots into AP mode next
+    // time (creds are already cleared) — instead of being in a "counter zeroed but
+    // creds still valid" inconsistent state.
+    memset(config.ssid, 0, sizeof(config.ssid));
+    memset(config.password, 0, sizeof(config.password));
+    EEPROM.end();          // close current handle before saveConfig opens its own
+    saveConfig();          // commits cleared credentials to flash
+
+    EEPROM.begin(512);
     rc.count = 0;
     EEPROM.put(RESET_COUNTER_ADDR, rc);
     EEPROM.commit();
     EEPROM.end();
 
-    // Clear WiFi credentials only — keep other settings
-    memset(config.ssid, 0, sizeof(config.ssid));
-    memset(config.password, 0, sizeof(config.password));
-    saveConfig();
-
     // Show reset screen
     display.clearDisplay();
     display.setTextColor(SSD1306_WHITE);
@@ -244,6 +250,11 @@ void ICACHE_FLASH_ATTR setupOTA() {
 void setup() {
   Serial.begin(115200);
   delay(100);
+
+  // Seed PRNG with hardware entropy: chip ID is unique per device,
+  // micros() varies on each boot due to power-on timing jitter
+  randomSeed(ESP.getChipId() ^ micros());
+
   Serial.println("\n\nTJ-56-654 NTP Clock with OTA v" FIRMWARE_VERSION);
   Serial.println("==========================================");
   Serial.println("Display: GM009605v4.3 OLED 128x64 (SSD1306 I2C)");