Add level 7: The Floor Map (complex nested data)

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>

src/levels/mod.rs

@@ -13,6 +13,7 @@
 pub mod l01_minimum;
 pub mod l02_kv;
 pub mod l03_dict;
+pub mod l07_complex;
 
 use serde::{Deserialize, Serialize};