Validate Snippet page declarations

[danielporterda]

What changed

Composes snippet-page validation against the separately validated condition page. It parses <Snippet>, validates basic attributes, resolves source/path attributes, applies repository/path safety and local-preview policy, maps condition context, checks candidate identity, and returns only fully validated snippets plus diagnostics.

It does not combine condition and snippet diagnostics into a whole-page entry point, add a CLI, resolve remote content, or compile MDX.

Why

Snippet declaration composition is independently reviewable from conditional composition and from the public validator command.

Validation

• python -m pytest tests/test_snippet_page_validation.py tests/test_condition_page_validation.py tests/test_candidate_condition_diagnostics.py tests/test_local_snippet_source_remediation.py (12 passed)
• python -m ruff check scripts/snippets/model.py scripts/snippets/page_validation.py tests/test_snippet_page_validation.py
• python -m mypy scripts/snippets/model.py scripts/snippets/page_validation.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.