[RELEASE] Credits & licenses final (CREDITS.md + ASSET_SOURCES.md)

## Goal
Lizenzsauberer Release.

## Tasks
- [ ] credits eintragen (CC-BY / credit-required)
- [ ] Quellen-Links prüfen
- [ ] in-game credits (optional v1.0)

## Acceptance Criteria
- [ ] alle verwendeten packs korrekt attribuiert

## Test Evidence
- [ ] PR review: credits ok

**Dependencies:** #33, #34, #35