feat: add com.mercadopago.payment.display extension

[gforgab]

What

Adds com.mercadopago.payment.display, a vendor-namespaced Checkout
extension that models in-session, out-of-band payment methods (e.g.
Pix) as a pure-render Action on top of the generic actions primitive
introduced in #582.

When the buyer picks such a method, the Business surfaces an inert display
artifact (QR image, copy-and-paste code, and/or hosted instructions) as an
outstanding Action on the checkout. The Platform renders it, the buyer pays in
their bank app, and the same Complete Checkout operation resolves by polling
Get Checkout — no redirect, no continue_url.

Why this shape

• Builds directly on the merged actions primitive (feat: extension-defined Actions primitive #582). Instance is
  { id, config? }; no required field — gating is expressed via a
  recoverable error Message whose path points at the action occurrence,
  per the outcome of the [RFC] Proposal: Modeling 3DS and Step-Up Actions in UCP #517 discussion.
• Vendor namespace, not core. Consistent with the Tech Council's steer on
  [RFC] Proposal: Modeling 3DS and Step-Up Actions in UCP #517 to keep core small and let independent, community-driven implementations
  land first. Namespace derives from the handler domain (com.mercadopago.*,
  matching com.mercadopago.checkout).
• Carries an explicit render/trust contract: display data only (no
  PAN/OTP/credentials), image is inert, code is display text, and
  instructions_url is the only loadable field (https origin allowlist).

Scope

• In-session methods that resolve in seconds (Pix). Out-of-session methods that
  settle after the session ends (boleto, cash vouchers) are out of scope and
  will be proposed separately at the order level (async-payments EP).

Changes

• source/schemas/shopping/payment_display.json — extension schema.
• docs/specification/payment-display.md — spec (discovery, schema,
  render/trust contract, resolution flow).
• mkdocs.yml — nav under Checkout Capability + llmstxt entry.
• .cspell/custom-words.txt — domain terms.

Related

• Builds on feat: extension-defined Actions primitive #582 (merged actions primitive).
• Sibling of feat: Add Payment Authentication Actions #458 (3DS/DDC Action extension).
• Context: [RFC] Proposal: Modeling 3DS and Step-Up Actions in UCP #517 (Actions / Step-Up RFC).

Opening as draft for early feedback on the extension shape and the
render/trust contract wording before finalizing.

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