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