feat: prepare private payments pocs

[Meyanis95]

Summary

• feat(pattern): Add stateless Plasma privacy pattern (Intmax-style)
• feat(approach): Expand private payments with Plasma and TEE approaches
• fix(use-case): Clarify private stablecoin problems and update pattern links
• fix(refs): Update private bonds PoC links to ethereum/iptf-pocs

Context

Preparing the private stablecoin use case and approach docs to support upcoming PoC work. Key changes:

1. New pattern: pattern-plasma-stateless-privacy.md — client-side proving with minimal on-chain footprint
2. Unified privacy approaches in approach-private-payments.md:
   • L1 Shielded Payments
   • Privacy L2 (Aztec)
   • Stateless Plasma (Intmax)
   • TEE-Based Privacy
3. Clarified problems in private stablecoins use case:
   • Problem 1: Payment privacy with regulatory compliance
   • Problem 2: User onboarding & fiat integration

Related

• PoC issues to be created in ethereum/iptf-pocs
• Differentiates from private bonds PoC (which uses shielding/Aztec for secondary trading)

Test plan

• Links resolve correctly
• Pattern follows template structure
• Approach and use case are coherent

[oskarth]

LGTM; please fix CI (including editing vale instructions if you think feedback is wrong)