Eliminates scope-dependent CDN variable bugs by introducing a `cdn` lookup
plugin that resolves paths and URLs at call-time using an explicit application_id
argument. Deletes the old cdn_paths.py and cdn_urls.py filter plugins and their
tests; adds a new unit test for the lookup plugin's _cdn_paths and _to_url_tree
helpers.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>