Custom Hegemony Formation

What it does
- Adds a major decision to create a dynamic hegemony-tier title for any independent ruler who meets the requirements.
- The new title uses CK3's real hegemony tier, becomes the ruler's primary title, and pulls personally held empire titles under it with set_de_jure_liege_title.
- Adds automatic empire integration through a nested on_title_gain hook, plus a manual fallback decision to re-integrate empire titles later.
- After creation, human players get a naming event that lets them keep the default hegemony name or enter a custom one immediately.

Requirements
- Independent ruler
- At least 2 personally held empire titles
- At least 15000 prestige
- At least 5000 gold
- Either realm_size >= 160 or at least 3 valid tributaries
- Valid tributaries are kingdom/empire-tier tributaries with feudal, clan, or administrative government

Compatibility notes
- Built against the local CK3 script set that includes hegemony tier, tributary contracts, create_dynamic_title, set_primary_title_to, set_de_jure_liege_title, and the de jure hegemony map mode assets.
- Uses additive decisions, scripted effects/triggers, localization, and a nested on_action hook only.
- If some future patch changes scripted title-gain behavior, the manual "Integrate Imperial Titles into the Hegemony" decision remains as a fallback.