freshness-and-drift-detection: drift finding — advisory suspect-artifact finding in doctor/review

[tcballard]

Sub-issue of #277 (freshness-and-drift-detection, ADR-093).

Corpus: rac/requirements/rac-drift-advisory-finding.md

Scope:

• New rac doctor finding, stable code (working name suspect-artifact): a validated relationship target's last-committed change is newer than the referencing artifact's — computed from the git-recency service (ADR-045) + the validated relationship graph (ADR-074) only.
• Advisory: warning severity in the existing finding shape through the existing diagnose seam; warning-only runs exit 0; rac review surfaces it beside the stale-corpus cadence advisory.
• Declared, resolvable artifact references only; external-reference sections (related tickets, verified by) excluded (ADR-087).
• Names the newer target + evidencing commits/dates as facts with a "review recommended" fix — no verdicts, no auto-fix (ADR-034).
• No-git/shallow history → no findings, no errors; goldens never depend on uncontrolled git state.
• No CI-failing mode in this phase (the gate form is a later-phase initiative under ADR-075); the finding shape admits additive code-scope extension once Epic: decision-to-code-proximity — decisions at the point of edit #273's declared scopes land — without renaming the stable code.

Tick on #277 when landed.