Scoped from the framework-pt keyset incident: adopt the `cashu` crate
(MIT, the crate CDK is built on) for the token codec, keyset ids, crypto,
DLEQ and NUT-13, while keeping ecash.rs's on-disk contract, MintClient's
Tor seam and error table, and our multi-mint routing. Records why the
full cdk WalletDatabase shim is the wrong trade over live funds, and how
ecash backup can derive from the node's existing seed.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
