Register AlgoVoi payment extensions in example extensions list (Settlement Attestation, Composite Trust Query, RFC 9421 Signing)

The AlgoVoi-authored payment extension suite covers four complementary extensions for A2A agents operating in payment contexts:

1. **Compliance Gate Extension** -- admission-time sanctions/KYC gate returning ALLOW / REFER / DENY (already in PR #1886)
2. **Settlement Attestation Extension** -- categorical post-settlement record: SETTLED / PENDING_FINALITY / REVERSED
3. **Composite Trust Query Extension** -- multi-issuer composite verdict: TRUSTED / PROVISIONAL / INSUFFICIENT_EVIDENCE / UNTRUSTED
4. **RFC 9421 Message Signing Extension** -- RFC 9421 (HTTP Message Signatures) binding for A2A task envelopes

All four are AlgoVoi-authored, Apache 2.0, with reference implementations on PyPI and npm, byte-for-byte cross-validated across 8 independent JCS implementations. Registering them in the example extensions table makes them discoverable to A2A implementors building payment-aware agents.

This issue tracks the registration of extensions 2, 3, and 4 (extension 1 is tracked by #1887).

Find the example extensions table in the repository documentation, likely in a README or docs directory. Add entries for the Settlement Attestation, Composite Trust Query, and RFC 9421 Signing extensions, following the existing format for the Compliance Gate Extension. Verify the table renders correctly and that the links to the reference implementations (PyPI/npm) are accurate.

Scritto dal modello di indicizzazione a partire dal testo della issue.