Add level 4: The Chest (lists)

Top-level `chest:` key with a 3-5 item sequence drawn from a small pool.
Per-seed deterministic via ChaCha8Rng XOR'd with 0x..04. Description
rendered with tera; flavor uses a 📦 emoji.

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 l04_list;
 
 use serde::{Deserialize, Serialize};