docs: codify Paykit issuer interoperability

[ovitrif]

Parent: #712
Source: Pubky marketplace Bitkit team brief
Counterpart: synonymdev/bitkit-android#1208

Scope

• Document network-correct on-chain identifiers, chain-independent Lightning identifiers, lowercase btc, and JSON endpoint payloads containing a non-empty value.
• Add focused tests for accepted and rejected request and endpoint shapes.

Acceptance criteria

• An issuer can implement the contract without reading Bitkit source.
• Test fixtures fail when Bitkit drifts from the documented contract.