Add corpus-by-seed robustness matrix

[almondsun]

Summary

• complete a preregistered 2-tokenizer × 2-corpus × 3-seed matrix with seven new fixed configs
• add a balance-checked matrix analyzer with paired BPC contrasts and corpus interactions
• move bounded run-artifact loading into the evaluation package and harden it against oversized or changing files
• document milestone 025, factorial interpretation, validation-selection bias, and the sealed-test next step

Result

ByteBPE512 beats character in all six same-seed comparisons. Mean candidate-minus-reference BPC is -0.061863 on Alice and -0.025203 on Peter Pan. The +0.036660 corpus interaction shows robust direction but corpus-dependent magnitude.

Validation

• exact 12-run matrix replay passed artifact, fingerprint, checksum, tokenizer, and seed-set validation
• make PYTHON=.venv/bin/python check: 155 passed, 91.26% coverage
• Ruff, strict mypy, compileall, Markdown links, and git diff --check passed
• make PYTHON=.venv/bin/python audit: no known third-party vulnerabilities
• direct CLI and oversized-artifact regression tests pass

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