chore(use-cases): audit private stablecoins and one hop out

[rymnc]

This pull request delivers a factual audit and regulatory update for the private-stablecoins entry and related jurisdictional documentation. It clarifies the regulatory landscape for stablecoins in Hong Kong, Singapore, and China, and improves the accuracy of surrounding patterns and references. The changes ensure that the documentation reflects the latest legal developments, corrects prior overstatements, and clarifies the applicability of patterns to specific jurisdictions.

Regulatory and Jurisdictional Updates:

• Updated the Hong Kong section to reflect the commencement of the Stablecoins Ordinance (Cap. 656), the licensing of stablecoin issuers by the HKMA, and clarified the separation of regulatory responsibilities between the HKMA (stablecoins) and SFC (virtual asset trading platforms). Also added references to the official registers and guidance. [1] [2]
• Revised the Singapore section to clarify that the MAS single-currency stablecoin framework does not have statutory force yet, and noted the opening of the September 2026 consultation on the implementing legislation. Added a reference to the MAS consultation.
• Updated the China section to clarify that BSN is not a channel for e-CNY, that e-CNY is a separate, centrally operated system, and improved the accuracy of the BSN reference. [1] [2] [3]

Pattern and Technical Documentation Corrections:

• Corrected and clarified privacy, security, and post-quantum mitigation details in the Plasma Stateless Privacy, Private Stablecoin Shielded Payments, DvP via ERC-7573, and Verifiable Attestation patterns. This includes more precise descriptions of threat models, guarantees, and standards status. [1] [2] [3] [4] [5] [6] [7]

Changelog and Review Dates:

• Added a changelog entry summarizing the factual audit and updated the last_reviewed dates across relevant files to September 2026. [1] [2] [3] [4] [5] [6] [7] [8]

These updates ensure that the documentation accurately reflects the current regulatory environment and technical standards for private stablecoins and related patterns.