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/config.h

@@ -9,7 +9,7 @@
 #include <Arduino.h>
 
 // Firmware version
-#define FIRMWARE_VERSION "1.9.8"
+#define FIRMWARE_VERSION "1.9.9"
 
 // OLED I2C Configuration
 #define I2C_SDA 0  // GPIO0 (I2C Data) - SWAPPED!
@@ -120,13 +120,11 @@ enum WeatherState {
   WEATHER_FAILED
 };
 
-// Async NTP state machine
+// Async NTP state machine — only IDLE and REQUEST_SENT are used
+// (response handler transitions back to IDLE directly on success or timeout)
 enum NTPState {
   NTP_IDLE,
-  NTP_REQUEST_SENT,
-  NTP_WAITING,
-  NTP_SUCCESS,
-  NTP_FAILED
+  NTP_REQUEST_SENT
 };
 
 // Async WiFi state machine

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)");

firmware/weather_clock/web_server.cpp

@@ -294,7 +294,8 @@ static bool isValidSSID(const String& s) {
     if (c < 0x20 || c > 0x7E) return false;  // non-printable
     if (c != first) allSame = false;
   }
-  return !allSame;  // reject "AAAAA...", "BBBBB...", etc.
+  // Single-char SSIDs ("A") are valid per 802.11. Only reject all-same for length>1.
+  return n == 1 || !allSame;
 }
 
 void ICACHE_FLASH_ATTR handleConfigSave() {
@@ -358,7 +359,9 @@ void ICACHE_FLASH_ATTR handleConfigSave() {
   }
   if (server.hasArg("ntp_interval")) {
     long ni = server.arg("ntp_interval").toInt();
-    config.ntp_interval = constrain(ni, 60L, 86400L);
+    unsigned long newInterval = constrain(ni, 60L, 86400L);
+    if (newInterval != config.ntp_interval) needsRestart = true;  // NTPClient constructed at boot with this
+    config.ntp_interval = newInterval;
   }
   if (server.hasArg("ntp_server")) {
     String n = server.arg("ntp_server");

firmware/weather_clock/wifi_manager.cpp

@@ -105,10 +105,14 @@ void ICACHE_FLASH_ATTR setupWiFi() {
     }
   }
 
-  // Try 2: If we have EEPROM credentials, try those
-  if (strlen(config.ssid) > 0 && strlen(config.password) > 0) {
+  // Try 2: If we have EEPROM credentials, try those (M2: support open networks)
+  if (strlen(config.ssid) > 0) {
     Serial.println("\nTrying EEPROM credentials...");
-    WiFi.begin(config.ssid, config.password);
+    if (strlen(config.password) > 0) {
+      WiFi.begin(config.ssid, config.password);
+    } else {
+      WiFi.begin(config.ssid);  // open network — no password
+    }
 
     int attempts = 0;
     while (WiFi.status() != WL_CONNECTED && attempts < 20) {