relationship-vocabulary: back-reference advisory — surface asymmetric edges (gap 6)

[tcballard]

Sub-issue of #236 (relationship-vocabulary, ADR-093). Bucket A — advisory only, no schema change.

Corpus: rac/requirements/rac-relationship-back-references.md (Proposed) — gap 6 of the traceability audit.

Scope:

• Relationships are declared on one side; a heavily-referenced artifact cannot advertise who cites it, and nothing warns when an expected reverse edge is absent. This is a detection gap, not a schema gap — no new sections, no new edge types.
• An advisory (doctor/review tier, never blocking) surfacing asymmetric edges, in the same never-blocking shape as the ADR-096/applies-to advisory precedent.
• Distinct from the unlinked-reference advisory (ADR-082), which covers prose mentions, not declared one-way edges.

Tick on #236 when the advisory ships.