docs: complete decision-flow public contract

[vitormattos]

Completes the remaining public-contract and interoperability requirements of #56.

Summary

• move emitDecision() to a dedicated optional DecisionNfseIssuerInterface capability so existing NfseClientInterface implementations remain source-compatible
• document the official decision-flow endpoint, payload, legal boundary, schema contradiction, IBS/CBS limitation, and no-blind-retry rule
• prove that an authorized bypass receipt returns to normal query() and cancel() APIs
• prove that the official substitution relationship is preserved inside the DPS embedded in the complete decision-flow NFS-e

Why the interface split

Adding the bypass method directly to NfseClientInterface would force unrelated third-party implementations to implement a legal-special-case capability. Keeping it separate preserves the ordinary client contract.

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