Add J.P. Morgan Online Payments checkpoint

[isomorphisms]

Adds a J.P. Morgan Online Payments checkpoint following the existing Reddit/Stripe pattern.

• inventories the complete current Online Payments surface: 16 method/path operations across payments, captures, refunds, verifications, and fraud checks
• pins the documentation pass to API specification release 2.19.0 and the documented mock /api/v2 base
• adds jpmorgan version, endpoints, url payment ID, fixture FILE, and first read-only payment ID command shapes
• adds a synthetic payment-response fixture and byte-stable TSV contract
• keeps OAuth token acquisition and money-moving writes out of the first slice
• leaves file input, JSON decoding, environment access, UUID generation, and caller-supplied ICU/Idric-Net headers as explicit named holes rather than hiding them behind curl/Python

The endpoint and sequencing audit is in checkpoints/jpmorgan/SURFACE.md.