New scripts/plot_substrate.py parses the hk_<mode> cumulative columns from a
multi-loophole substrate run (one log, K interleaved modes) and draws one
learning curve per mode with first_step onset dots and direct end-labels.
plot_emergence.py can't do this (it groups logs by a single --env-mode).
Figure shows the headline: vanilla GRPO learns file_marker/run_tests/
stdout_marker/sentinel, eq_override flat at 0 (never).
Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>
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