feat(editor): add WLDN (Learning Notification) — 105th open format

Novel replacement for the hardcoded server-side
milestone messages that fire when a player crosses a
progression threshold ("You can now learn Apprentice
Riding" at level 20, "Dual specialization is now
available", "You have unlocked the auction house"). Each
entry binds one trigger condition (LevelReach /
FactionStanding / ItemAcquired / QuestComplete /
SpellLearned / ZoneEntered) to a delivery channel
(RaidWarning banner / SystemMsg / Subtitle / Tutorial
popup / MOTDAppend) and an optional fanfare sound.

The triggerValue field is polymorphic — its semantics
depend on triggerKind. The validator enforces per-kind
ranges: LevelReach 1-80 (current cap), FactionStanding
+/-42000 (Hated to Exalted bounds), ItemAcquired/
QuestComplete/SpellLearned/ZoneEntered must be a
positive id (>0). This is the first format to use
per-trigger discriminated value validation.

Three preset emitters: makeLevelMilestones (5
LevelReach unlocks at canonical thresholds 20/30/40/60/
80), makeAccountUnlocks (4 mixed-kind notifications:
first-mail tutorial gated to <2hr playtime, Stormwind
auction-house location subtitle, dual-spec activation
on spell-learn, transmog vendor unlock on quest-
complete), makeReputation (3 FactionStanding milestones
at Honored/Revered/Exalted standings).

minTotalTimePlayed gates first-time-only tutorials —
the auction-house location subtitle fires only for
characters with <2hr total time so veterans don't get
spammed.

Format count 104 -> 105. CLI flag count 1155 -> 1160.

tools/editor/cli_arg_required.cpp

@@ -322,6 +322,8 @@ const char* const kArgRequired[] = {
     "--gen-spm", "--gen-spm-raid", "--gen-spm-env",
     "--info-wspm", "--validate-wspm",
     "--export-wspm-json", "--import-wspm-json",
+    "--gen-ldn", "--gen-ldn-account", "--gen-ldn-rep",
+    "--info-wldn", "--validate-wldn",
     "--gen-weather-temperate", "--gen-weather-arctic",
     "--gen-weather-desert", "--gen-weather-stormy",
     "--gen-zone-atmosphere",