feat(pay): checkout core — invoice model, payment orchestration, mock wallet, honesty report

The core composes only the four shipped wallet actions (shield / private
transfer / unshield / swap) so the mainnet path never depends on unpublished
proving or discovery endpoints. MockWallet exercises the full flow, including
shield-then-pay and every failure branch, with no extension or funds.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>