docs(point-of-sale): pin paymentvalidations to tender-confirm semantics

[aaronschubert0]

What

Docs-only follow-up to #4611: pins PaymentValidationsEvent's dispatch timing, which was settled after that PR merged.

• Fires when a tender is confirmed but not committed — after the amount is entered, before the payment is recorded (previously read "when staff selects a payment method", which was the pre-settlement wording).
• One event per tender attempt; split payments dispatch one event per tender, each carrying its own amount.
• Replaces the rejected two-event proposal (method-selected + post-tender).

Confirm-time is also why amount is per-tender meaningful: at selection time only the outstanding total exists; at confirm time the event carries the amount this tender actually charges.

References

• Types: feat(point-of-sale): add paymentvalidations intercept types #4611 (merged) · Release: Version Packages (2026-10-rc) (rc) #4622 (2027.0.0-rc.5)
• API contract: Shopify/ui-api-design#1557
• POS host implementation (gates at useCashPaymentSubmission, before addPayment): shop/world#997831 → shop/world#1000498 → execution PR to follow

[aaronschubert0]

Merge activity

• Aug 18, 9:47 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Aug 18, 9:48 AM UTC: Graphite rebased this pull request as part of a merge.
• Aug 18, 9:49 AM UTC: @aaronschubert0 merged this pull request with Graphite.