[ychampion]

When "smartquotes" was enabled in myst_enable_extensions, create_md_parser enabled markdown-it-py's smartquotes rule with its hardcoded English quote set (“”‘’). During parsing, ASCII quotes were converted to English curly quotes before Sphinx's SphinxSmartQuotes or Docutils' SmartQuotes transform ran, ignoring Sphinx's language setting and Docutils' --language, --smart-quotes, and --smartquote-locales settings.

Changes

• Add pure helpers resolve_smartquotes() and document_smartquotes() in myst_parser.parsers.mdit that resolve the 4-item (open_double, close_double, open_single, close_single) sequence using Docutils' smartchars.quotes table, BCP 47 subtag fallback (normalize_language_tag), smart_quotes="alt" (-x-altquot), and smartquotes_locales overrides without mutating global Docutils state, falling back to English typographic quotes when unset or unsupported.
• Accept an optional keyword-only quotes override in create_md_parser() and pass document_smartquotes(document) from both MystParser.parse() (Sphinx) and Parser.parse() (standalone Docutils) whenever "smartquotes" is active (including per-file front matter and nested {include} renders).
• Document localized quote behavior under docs/syntax/optional.md and add unit and integration tests for standalone Docutils and Sphinx builds.

Fixes #806