[G1-GATES] Every guardrail flips from report mode to a gate: tests clone duplication ≤ 5% #499

Part of #350, the quality-refactoring roadmap (road to 1.0, step 3).

Goal: Every guardrail flips from report mode to a gate: tests clone duplication ≤ 5% (enforced since #474), no assertion that prints only false, test files ≤ 600 lines.

Lane: Lead + Tests. Status: active. Merged so far: #474.

Every PR of this epic starts its title with [G1-GATES]. The finding issues it closes are linked as sub-issues; this issue closes when the epic's last PR merges and master CI is green.