Add snippet validation CLI

[danielporterda]

What changed

• adds a command-line entry point for validating authored *.source.mdx files
• prints existing page/line diagnostics to stderr and exits nonzero on errors
• supports --allow-local for preview-only validation

Why

This exposes the validation layers from the preceding PRs without yet coupling them to npm, CI, compilation, or preview behavior.

Validation

• 7 focused pytest tests
• Ruff
• mypy with explicit package bases
• git diff --check

Not included

• npm scripts
• CI gating
• snippet resolution or compilation