[7-falseparfait]

Description

closes #716
currently, parsing descriptors containing secret keys such as xprv and tprv into the existing public descriptor type requires a secp256k1 signing context. the parser derives public keys and stores the corresponding secrets in a keymap.
this pr introduces secretdescriptor, a wrapper around descriptor that provides:

• context-free parsing and serialization: parse descriptors containing private or public keys without a secp256k1 context or key derivation.
• typed key validation: preserve key metadata during parsing so descriptor rules are enforced, including rejecting uncompressed keys in segwit descriptors such as wpkh.
• public and mixed-key conversions: convert to and from a public descriptor and keymap; keys absent from the map remain public. converting secret keys to public keys still requires a signing context.
  unit tests cover secret-key parsing, wif formatting and checksums, segwit key validation, and public-only and mixed-key conversions.

[7-falseparfait]

fixed formatting, all ci checks should be passing now