Top-level `chambers:` mapping; each chamber name maps to an item list.
2-3 chambers, 2-3 items each, drawn from typed pools. Deterministic per
seed via ChaCha8Rng XOR'd with 0x..05.
Not wired into levels::registry() yet — integration belongs to a follow-up.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
