`vault:` with four entries — decimal int gold, decimal int silver,
fractional float experience, and an ISO-8601 date string. Description
hints at hex/octal/exponent equivalents the player can write — any
form parsing to the same numeric value passes.
All four values randomised per seed (ChaCha8Rng XOR'd with 0x..0A).
Not wired into levels::registry() yet — integration belongs to a follow-up.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
