Add Frontier decoder results

[aleverrier]

Summary

• add the Frontier decoder to the existing BB72, gross-code, rotated-surface-code, and 2D-color-code entries
• cite Leverrier and Urbanke, arXiv:2606.20513, and link the open-source implementation
• report the paper's code-capacity and detector-error-model results with their noise-model and decoding-scope qualifiers
• add Anthony Leverrier to the code-contributor registry

Result scope

The BB entries identify the per-side detector error model matrix sizes and full-FER comparisons. The rotated-surface entry distinguishes correlated-Pauli code capacity from the circuit-level memory-Z experiment and its fitted error-suppression coefficient. The color-code entry identifies the hexagonal-code distances and code-capacity bit-flip setting.

This does not claim a DecoderBench result; the paper used its own benchmark corpora.

Validation

• parsed all five changed YAML files with PyYAML 6.0.3
• ran the repository spell checker on all four changed code entries: zero issues
• ran git diff --check

The repository's pull-request CI will perform the full site build and citation-resolution check.