The route2 tau-gate anchored on (teacher OR hacked_E student). hacked_E is the
run_tests detector; it cross-fires <=1.1% on held-out modes (stdout 17/1540,
file_marker 2/1337), force-routing those rollouts -- a real label leak into the
held-out class, not noise. Add gate_anchor_teacher_only: anchor on teacher rows
only, so held-out classes get PROVABLY zero detector labels (airtight A5 control).
Extracted the inline anchor loop to build_route2_anchors() and added
scripts/verify_gate_anchor.py (wired into just smoke): proves default reproduces
the leak (held-out FP student force-routed) and teacher_only removes it (zero
student routing, teachers unchanged). 9/9 assertions pass.
Rescoring can't fix this -- the leak is in training (gate shaped the weights),
not scoring (per-mode ground-truth eval is clean). Retrain is the only path; the
A5 run saved no per-eval checkpoints anyway.
Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>
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