docs: add English factor handbook

[redamancy231-create]

What changed

• add docs/factor_handbook_en.md, an English companion to the existing Chinese handbook
• preserve all 204 handcrafted factor identifiers and family groupings
• document that compute_legacy_set(panel) returns the unified 213-feature set: 204 handcrafted factors plus 9 selected Alpha101 factors
• add research-only caveats and clarify that registered code is authoritative
• add English/Chinese handbook links to the main README

Maintenance cleanup

The branch was rebuilt on the current main. Translation prompt/output artifacts, duplicate #35 history, unrelated validation-script changes, and the ineffective warning in the shadowed src/mlquant/features.py module were excluded.

[initial-d]

Thanks for preparing the English handbook. I amended the PR before merge to keep it focused and maintainable:

• rebuilt the branch on the current main and removed duplicate fix: harden neutralization and Baostock data handling #35 history
• excluded translation prompt/output artifacts and the unrelated validation-script change
• excluded the src/mlquant/features.py warning because that file is shadowed by the mlquant.features package and is not the public import path
• verified that all 204 handcrafted factor identifiers match the existing Chinese handbook; the additional 9 registered alpha_* factors make the default 213-feature set
• corrected the usage example from 204 to 213 default features
• added research caveats, a code-is-authoritative note, and bilingual README links
• cleaned the commit metadata

This is now a two-file documentation contribution. CI intentionally skips Markdown/docs-only changes under the repository workflow, so validation used factor-name parity, link targets, and a clean Markdown diff.