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>

CHANGELOG.md

@@ -5,6 +5,30 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.9.9] - 2026-05-19
+
+### Fixed
+
+- **Factory reset atomicity**: cleared WiFi credentials are now committed BEFORE
+  the reset counter is zeroed. If power fails between the two writes, the device
+  still boots into WiFiManager AP on next start (cleared creds win over stale counter)
+  instead of being stuck in an inconsistent state
+- **`isValidSSID` rejected single-char SSIDs**: length-1 networks like "A" were
+  incorrectly flagged as all-same garbage. Now allowed (per 802.11 spec)
+- **`ntp_interval` config changes ignored until reboot**: `NTPClient` is constructed
+  in `setup()` with this value and never re-initialized. Now triggers `needsRestart`
+  when the interval actually changes
+- **Open WiFi (no password) failed to connect at setup** (M2): Try-2 block in
+  `setupWiFi()` required both ssid and password; now falls back to `WiFi.begin(ssid)`
+  when password is empty
+- **Unseeded PRNG, identical dissolve pattern every boot** (M4): `randomSeed()`
+  now called with `ESP.getChipId() ^ micros()` — varies between devices and boots
+
+### Removed
+
+- **Dead `NTPState` enum values** (M3): `NTP_WAITING`, `NTP_SUCCESS`, `NTP_FAILED`
+  were defined but never assigned. Removed to reduce code surface area
+
 ## [1.9.8] - 2026-05-19
 
 ### Fixed