Add event prerequisite gates

docs/GODOT_4_6_MIGRATION.md

@@ -49,6 +49,7 @@ powershell -NoProfile -ExecutionPolicy Bypass -File tools\check_godot46_readines
 - Named equipment rewards such as Yitian Sword and Dragon Spear validate, appear with compact rarity tags in reward/inventory/equipment text, and remain ordinary stackable inventory stock until unique item instances are implemented.
 - Hovering an occupied tile shows the unit's class, movement type, AGI, movement, range, equipped weapon/armor/accessory, and a HUD portrait thumbnail when no unit is selected, with empty gear slots shown as `-`.
 - The battle HUD and briefing objective panels split objective, progress, and risk details, including pending unlocks, destination reach status, defeated enemy counts, named protected-unit or commander safety, and turns remaining.
+- Scenario event `when.after_event` prerequisites keep follow-up events from firing before their one-shot setup event.
 - `set_objective` scenario events show concrete objective-update logs, play a UI confirmation sound, refresh the HUD immediately, and briefly show the objective notice panel.
 - `withdraw_unit` and `withdraw_units` scenario events remove living targets from the battlefield without marking them defeated.
 - Movement-triggered supply cache events grant battle inventory items with pickup feedback, and the items persist only after victory.