[P4][SHOP-03] Checkout and payment integration

Programme: #1
Epic: #27

## Objective
Support purchase execution through approved merchant/payment routes with fresh quotes, exact account/delivery confirmation, trusted approval and post-commit verification.

## Scope
- Quote preparation/refresh from supported commerce connector/protocol.
- Merchant/seller, product/variant, quantity, currency, item price, shipping, tax, fees and destination binding.
- Merchant/PSP/tokenized payment handoff; avoid handling raw card data where possible.
- Trusted purchase confirmation through TX-01.
- Commit via supported API/protocol; supervised Original-view handoff where custom checkout is unavailable.
- Provider order/receipt verification and TX-02 reconciliation.
- Support-matrix integration with CONN-04 production approvals.

## Safety rules
- Any material total/item/destination/account change invalidates approval.
- Missing tax/shipping means total is not yet fully known.
- Ambiguous commit is never automatically retried.
- Visual checkout automation is not marketed as equivalent to an API-backed transaction.

## Acceptance criteria
- [ ] Production-enabled routes have documented live provider access/approval.
- [ ] Approval is bound to exact merchant/seller/item/variant/destination/currency/total or configured ceiling.
- [ ] Material quote changes invalidate approval and require refresh/review.
- [ ] Commit produces verified provider order/receipt or explicit NeedsReconciliation.
- [ ] No ambiguous purchase fixture duplicates an order.
- [ ] Unsupported merchant checkout hands off authentically to Original mode with truthful state-transfer limits.

## Dependencies
- SHOP-01
- TX-01
- TX-02
- CONN-04

**First phase:** P4
**Maturity target:** P7
**Owner:** connectors-domains

No implementation files or entry points are named. Start by reading SHOP-01, TX-01, TX-02, and CONN-04, then trace the supported commerce connectors and payment routes; done means the listed approval, quote-refresh, verified-order, reconciliation, duplicate-prevention, and Original-mode handoff criteria are met.

Written by the indexing model from the issue text.