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>
