[Intent] Register Compliance Gate Extension URI in example extensions list

## Intent

Register the AlgoVoi-authored **Compliance Gate Extension v1** in the
example extensions list at `docs/topics/extensions.md`, alongside the
existing entries (Secure Passport, Timestamp, Traceability, AGP).

## Extension URI

`https://docs.algovoi.co.uk/compliance-gate-v1` (AlgoVoi-controlled)

## Summary of the extension

A Method Extension defining the `compliance/gate` RPC method. The
method returns a categorical admission-time verdict (`ALLOW` / `REFER`
/ `DENY`) as a seven-field Compliance Receipt canonicalised under
RFC 8785 (JCS) with SHA-256 binding to the bound Payment Mandate or
Cart Mandate.

The extension enables agent-to-agent payment authorisation flows to
obtain a regulator-recognised admission-time decision (sanctions
screening, jurisdiction filtering, KYC verification) before any payment
mandate is constructed.

## Production reference

The endpoint shape and verdict format are generated in production by
the AlgoVoi facilitator at
[`api.algovoi.co.uk/compliance/screen`](https://api.algovoi.co.uk/compliance/screen),
live since 2026-05-06 across eight chain families. Public audit
verifier: [`docs.algovoi.co.uk/audit-verifier`](https://docs.algovoi.co.uk/audit-verifier).

## Companion IETF Internet-Draft

[`draft-hopley-x402-compliance-receipt`](https://datatracker.ietf.org/doc/draft-hopley-x402-compliance-receipt/)
(Independent Submission, Informational, sole AlgoVoi authorship).

## Reference implementation

AlgoVoi-authored, Apache 2.0:

- [`algovoi-substrate`](https://pypi.org/project/algovoi-substrate/) on PyPI
- [`@algovoi/substrate`](https://www.npmjs.com/package/@algovoi/substrate) on npm

Canonicalisation layer cross-validated 192/192 byte-for-byte across
eight independent language implementations (Python, TypeScript, Go,
Rust, Java, PHP, .NET, Ruby).

## Implementation PR

PR #1886 adds a single row to the example extensions table in
`docs/topics/extensions.md` pointing at the AlgoVoi-controlled URI.
No code changes. No samples in the A2A tree. No changes to existing
entries.

-- AlgoVoi (chopmob-cloud)
https://docs.algovoi.co.uk/acquisition