chore(approaches): Add I2U analysis to existing approaches

[Meyanis95]

Most approaches are written from an institution-to-institution (I2I) perspective — two sophisticated counterparties trading privately. When an end user enters the picture, the trust assumptions change: the institution holds structural power, and the user has no leverage, no negotiating power, and no contractual recourse.

Existing approaches should be updated with a section analyzing end-user implications.

Scope

Add an End-user considerations section to each approach covering:

• What can the end user do if the operator censors, surveils, or goes offline?
• What visibility does the operator have over user activity?
• What exit paths exist independent of operator cooperation?

Approaches to update

• approach-private-payments.md
• approach-private-bonds.md
• approach-private-derivatives.md
• approach-private-trade-settlement.md
• approach-white-label-deployment.md
• approach-private-fx.md
• approach-private-repo.md
• approach-private-rwa-tokenization.md

approach-private-identity.md and approach-private-money-market-funds.md already cover this and can serve as reference.