The other half of the no-cheat family (sibling of the gate-anchor leak). Asserts
on the real out/pools/substrate/partition.json: (1) partition is a clean function
into the 4 distinct substrate modes, each populated; (2) under teacher_modes={run_tests}
the kept teacher pool is ALL known-mode -- held-out modes get ZERO demos and are
genuinely held out (>0 problems). Vibe-check, not a theorem; wired into just smoke.
6/6 pass.
Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>
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