Implement reverse-domain issuer proof validation

## Objective

Implement reverse-domain issuer identifier proof using DNS or well-known web-origin validation.

## Scope

This issue proves that a caller controls the domain encoded by a reverse-domain issuer identifier such as `com.example.product`.

## Deliverables

- [ ] Define the canonical mapping from reverse-domain issuer id to DNS/web origin.
- [ ] Implement DNS TXT validation and/or HTTPS well-known validation according to the ADR.
- [ ] Validate the expected nonce, issuer id, and challenge id.
- [ ] Add resolver/client timeout, retry, and failure behavior.
- [ ] Add tests for valid proof, wrong domain, stale nonce, DNS/web failures, and malformed reverse-domain identifiers.
- [ ] Document operator and issuer-facing validation requirements.

## Acceptance Criteria

- [ ] A caller can claim a reverse-domain issuer id only by proving control of the corresponding domain.
- [ ] Domain proof cannot be reused across issuer ids or challenges.
- [ ] Network failures fail closed and are observable.

## Dependencies

- Depends on the ADR/API contract issue.
- Depends on the challenge lifecycle issue.

## Related Issues

- Parent: #187