feat:Add third-party beneficiary API integration guide

[noahmercy713-ship-it]

Summary

Changes

Testing

Parity checklist

This project maintains a feature-parity table in PARITY.md that
tracks which features are implemented on iOS vs Android. Please answer the
questions below before requesting review.

• This PR does not add, change, or remove any user-facing feature on either
  platform — no PARITY.md update needed.

  — OR —

• This PR adds/changes/removes a user-facing feature. I have updated PARITY.md:

  • Updated the status symbol(s) for the affected row(s).
  • Added or updated "Notes" if the implementation is partial or has caveats.
  • Removed or updated any rows in the "Known gaps" table that this PR closes.

Related issues

closes #300
closes #301
closes #302

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