refactor(xtask): event-envelope-aware scanning in ADR-004 audit

Plan originally called for parsing each service's events.rs separately and extracting fields of each published envelope. Current scanner treats those files like any other source. Published-payload regressions would still be caught by existing patterns, but a dedicated envelope-aware pass would be sharper.

Extracted from adr-004-ssa-authorization-audit.adoc § Potential Improvements.