Add cross-corpus robustness milestone

[almondsun]

Summary

• add a bounded deterministic Project Gutenberg body extractor with contract tests
• add fixed Peter Pan character and ByteBPE512 experiment configs
• document milestone 024 results, provenance, generation diagnostics, and external-validity theory

Result

ByteBPE512 reaches best full-validation BPC 2.153930 versus 2.172051 for the character control on near-size-matched Peter Pan, a narrow 0.83% advantage. The direction replicates the Alice result, but one seed does not establish a stable effect size.

Validation

• make PYTHON=.venv/bin/python check: 146 passed, 91.51% coverage
• make PYTHON=.venv/bin/python audit: no known vulnerabilities
• deterministic extraction checksum reproduced
• git diff --check

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