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).