👌 Add mapped token subtype for fence and HTML blocks

[adamtheturtle]

Summary

Implements the mapped-token subtype approach for #430. MappedToken requires a source map at construction and types map as list[int]; ordinary Token.map remains optional. The built-in fence (including make_fence_rule variants) and HTML-block rules now emit it via StateBlock.push_mapped(). Plugins can opt in.

Consumers narrow with isinstance(token, MappedToken); checking token.type alone is not sound because token types are extensible. This change is deliberately scoped to the two built-in rules motivating #430, not every token that happens to have a map. Existing Token construction, serialization shape, and value equality remain compatible.

Validation

• Python 3.10, 3.11, 3.12, and 3.13: 1,006 tests passed on each
• mdit-py-plugins suite against this branch: 511 passed
• pre-commit run --all-files: passed, including strict mypy
• git diff --check: passed

The upstream GitHub Actions CI and fuzzing runs are awaiting maintainer approval for this fork PR. Local clean docs builds hit the pre-existing docs/conf.py use of sphinx.ext.apidoc.OPTIONS, which is absent in current Sphinx 8/9; the Read the Docs check is reported separately by GitHub.