Discover authored snippet source pages

[danielporterda]

What changed

Adds deterministic discovery for authored *.source.mdx pages:

• recursively finds source pages below directory inputs
• accepts explicit source-page file inputs
• ignores ordinary MDX files and missing paths
• resolves, deduplicates, and sorts results

This PR intentionally does not parse <Snippet> or <IfVersion> syntax.

Why

Source-page opt-in is a separate contract from the directives that may appear inside those pages. Isolating discovery makes the file boundary independently reviewable and testable.

Validation

• python -m pytest tests/test_snippet_source_page_discovery.py (3 passed)
• python -m ruff check scripts/snippets/discovery.py tests/test_snippet_source_page_discovery.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.