Payment request consolidation (and extra fees for non-preferred mints)

[robwoodgate]

Consolidates and Supersedes: #355, #380

• Cashu-TS: feat: nut-18 mint preferences (ms, fr, sm) cashu-ts#683
• CDK: TBC
• Nutshell: TBC

Summary

Consolidates the two open NUT-18 payment request proposals around mint preferences, strict mint lists, fee reserve signalling, and required mint payment methods.

This PR keeps the existing m mint list as the single mint-list field, and adds small companion fields to clarify how that list should be interpreted.

Changes

• Retains ms (mint_strict) to signal whether the m mint list is strict or preferred from feat: preferred mints #380:
  • absent / true: receiver only accepts proofs from listed mints
  • false: listed mints are preferred, but other mints may be accepted
• Retains sm (supported_methods) to require that the mint used by the payer supports at least one listed payment method, e.g. bolt11, bolt12, onchain from feat(nut-18, nut-26): add preferred_mints to payment requests #355
• Adds fr (fee_reserve) to signal an additional amount the payer MUST include when paying from a non-preferred mint, to cover potential swap/melt fees
• Updates NUT-26 TLV tags for the new fields
• Adds test vectors covering the combined behaviour

Rationale

There were overlapping proposals for preferred mints and strict mint lists. Instead of introducing a separate preferred mint list, this keeps the request format minimal:

• m remains the mint list
• ms defines whether that list is strict or advisory
• fr gives wallets a way to compensate for extra fees when ignoring the preference
• sm lets receivers express the mint capabilities required for the payment flow

This avoids two competing mint-list fields while covering the three desired behaviours.