Parse inline Snippet tags

[danielporterda]

What changed

Parses well-formed, self-closing <Snippet ... /> tags into a small syntax model containing raw attributes and source spans.

The parser:

• supports multiline tags and quoted attributes
• preserves byte offsets plus line and column information
• ignores examples inside fenced code, inline code, and MDX comments
• rejects duplicate attributes and malformed or non-self-closing tags

This PR intentionally does not parse <IfVersion>, interpret snippet source references, or perform semantic validation.

Why

Snippet syntax is independently reviewable from release-condition syntax and from the rules applied to parsed attribute values.

Validation

• python -m pytest tests/test_snippet_tag_parser.py tests/test_snippet_source_page_discovery.py (9 passed)
• python -m ruff check scripts/snippets/model.py scripts/snippets/syntax.py tests/test_snippet_tag_parser.py
• python -m mypy scripts/snippets/model.py scripts/snippets/syntax.py
• git diff --check

[danielporterda]

Closing this draft because the inline release-aware snippet stack is being retired. The branch remains available if its implementation is needed later.