docs: translate Chinese comments and docstrings into English

[humdrum00001010]

Chinese-only comments and docstrings make the PDF analysis and document conversion implementation difficult to follow for English-speaking contributors. Translate them into English across Python, Rust, tests, tools, workflow comments, CSS, and ignore-file comments. This is a machine-assisted translation with additional manual wording review of the PDF analysis entry points, EPUB renderer, and workflow comments. Preserve executable production code, identifiers, runtime strings, Chinese fixtures, and localized documentation.

The repository has no CONTRIBUTING.md or PR template. One existing architecture test explicitly required Chinese characters in renderer docstrings; make that guard language-neutral while retaining its requirement for responsibility descriptions. This is the only intentional executable-code change, and it is confined to the test guard.

Validation:

• Python suite with the CI sparse-table exclusion: 7,296 passed, 852 skipped (Python 3.14, macOS arm64, Python backend, PDFium 5.10.1).
• cargo test -p docvortex-core --locked: 15 passed; Rust formatting passes.
• Ruff 0.14.2 lint and formatting checks pass.
• AST/token comparisons confirm executable production code is unchanged; the ignore patterns are unchanged too.
• A comment/docstring scan finds no remaining Chinese comments or docstrings in the code files.
• A real PDF-to-EPUB smoke conversion produces a valid EPUB container and preserves the sample paragraph.

Full cross-platform and Rust-backend validation remains to the upstream CI matrix.

[humdrum00001010]

It's a PR and also an idea proposal, if you dislike it, close!