refactor(xtask): context-aware identifier extraction in ADR-004 audit scanner

The ADR-004 audit scanner (cargo xtask compliance audit-data-tenancy) treats use canopy_common::fti_audit::scrub_fti_fields; the same as a struct field fti_audit: String. A richer pass that distinguishes Rust use / mod keywords from field / column declarations would eliminate false positives like the fti_audit import.

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