v0.7.0 — UCP PSP-neutral payment seam

Steelyard v0.7 generalizes UCP payment wiring beyond Stripe-specific Shared
Payment Tokens. Merchant PSP adapters now declare neutral payment capabilities,
UCP discovery derives payment_handlers from those capabilities, and the buyer
matches wallet issuers to advertised available_instruments instead of falling
back to implicit Stripe behavior. Stripe remains supported through
shared_payment_token, but UCP can now complete with another issuer-defined
instrument type.

The release adds a guarded reference PSP and buyer issuer for local interop and
demo validation. The reference path signs delegated_payment_token handles and
verifies merchant, checkout, transaction, amount, currency, handler, instrument,
signature, and expiry before capture. Coffee-shop validation now runs the same
catalog through Stripe-backed and reference-backed UCP checkout configs and
compares receipt shape and order outcome.

ACP is unchanged in scope: it remains intentionally direct Stripe SPT-only in
v0.7 and rejects non-shared_payment_token wallet issuers before minting. The
new docs describe the adapter boundary, the reference PSP guard rails, neutral
Stripe error mapping, and the UCP/ACP split.