Add level 10: Vault Ledger (dynamic values)

`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>

src/levels/mod.rs

@@ -19,6 +19,7 @@ pub mod l06_anchors;
 pub mod l07_complex;
 pub mod l08_tags;
 pub mod l09_operators;
+pub mod l10_dynamic;
 
 use serde::{Deserialize, Serialize};