Top-level `floor:` int + `rooms:` mapping where each room carries a
type, a locked flag, an exits list, and a contents list. Two rooms
picked from a name/kind pool; exits and contents drawn from typed
pools. Deterministic per seed (ChaCha8Rng XOR'd with 0x..07).
Not wired into levels::registry() yet — integration belongs to a follow-up.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
