test: pin MIME and response render parity

[narutamaaurum]

Summary

• add one canonical fixture that is rendered through MIME"text/html", MIME"text/plain", and html_response(...).body
• assert the HTML sink and HTTP response body are byte-identical
• assert the plain-text sink stays HyperSignal.Element: plus the same HTML bytes
• update the changelog note to call out the three-way parity fixture explicitly

Validation

• Not run locally in this cron environment (julia is not installed here, and the filesystem is currently full), so this PR relies on the repo CI matrix for verification.

Closes #4