Rename Compare to Benchmark in English documentation

[markusthemaker]

Summary

• rename the formal user-facing Compare result type to Benchmark throughout docs/doc_en.py, while retaining ordinary comparison language and compatibility identifiers
• expand the Operator Guide's experimental playbook to describe reach, Decode Rate, successful SNR, paired Delta SNR, one-sided evidence, geography, time and evidence support
• strengthen the distinction between direct WSPR observations, defensible conclusions and causal interpretation
• preserve the existing documentation structure and anchors

Compatibility

Internal schema and export identifiers such as results_view.compare, compare/, compare_evidence_figures and compare_evidence_recipes remain unchanged and are documented as compatibility names.

Validation

The branch automation validates Python syntax, unchanged documentation anchors, intentional remaining uses of ordinary Compare, required Benchmark headings and a documentation-only final diff.