Parse pull request snippet refs

[danielporterda]

What changed

Adds a pure parser for candidate snippet sources expressed as canonical GitHub pull-request URLs.

It accepts only https://github.com/<owner>/<repo>/pull/<positive-number> and returns repository identity plus the PR number as a typed reference. Alternate hosts, zero or non-numeric IDs, trailing slashes, subpages, and query strings do not parse.

This PR does not parse the candidate file path, require an enclosing <IfVersion>, or compare the candidate with a condition. Those are separate semantic layers.

Why

Candidate references have a different lifecycle and shape from immutable blob references, so their syntax should be independently reviewable.

Validation

• python -m pytest tests/test_pull_request_snippet_source_parser.py tests/test_immutable_snippet_source_parser.py (14 passed)
• python -m ruff check scripts/snippets/model.py scripts/snippets/references.py tests/test_pull_request_snippet_source_parser.py
• python -m mypy scripts/snippets/model.py scripts/snippets/references.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.