Parse immutable snippet source refs

[danielporterda]

What changed

Adds a pure parser for immutable snippet sources expressed as complete GitHub blob URLs.

It accepts only https://github.com/<owner>/<repo>/blob/<40-character-commit>/<path>, normalizes the commit hash to lowercase, and returns repository, commit, and path as a typed reference. Branch names, abbreviated hashes, missing paths, query strings, fragments, and non-GitHub hosts do not parse.

This PR does not parse candidate PR refs or local refs, check repository membership, or validate whether the parsed path is safe.

Why

Each supported source-reference form should be independently reviewable before they are combined into the <Snippet> semantic validator.

Validation

• python -m pytest tests/test_immutable_snippet_source_parser.py (7 passed)
• python -m ruff check scripts/snippets/model.py scripts/snippets/references.py tests/test_immutable_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.