Virtual URI mappings are extracted from core, and moved into their own registry (MAGNOLIA-3349); their API got few key improvements as well (MAGNOLIA-7016).

This epic tracks tasks following up to this extraction:

• VH assistance for module to upgrade themselves (thus leaving as few tech-debt behind)
• Double-validating compatibility w/ downstream usages in other modules, including a closer look/status at blossom and vanity-urls.
• New app possibility