Confine routing-field templates to prevent injection

Editor assessment

This commit patches CVE-2026-77621. The breaking bang is correct and the sinks scope fits, but the message has no body: there is no description of the template-injection risk, what behavior breaks, or how embedders should migrate. The subject itself is clear and specific, so the message is scannable, but a breaking change without impact text is incomplete by the rubric. Style-wise the breaking bang and sinks scope are correct; a breaking change with no body is the core weakness, because neither the break nor the migration is stated.