Add draft-aphelionz-marginal-fee: Reduce Marginal Fee to 1000 Zatoshis

Adds a draft ZIP proposing a reduction of ZIP 317's `marginal_fee` from 5,000 to 1,000 zatoshis per logical action. The conventional fee for a minimal 2-action transaction goes from 10,000 to 2,000 zatoshis; no other ZIP 317 parameter or formula changes. This is a wallet conventi