Allow template overrides to reference originals

Have some path for templates that always resolves to embedded ("originals/" or something like that), to allow overriding templates to derive from them or reference them for functionality, etc.

No file, test, or entry point is named. Start by locating the template loading and override-resolution code, then determine how an embedded original could be addressed from an override. Done means an override can reliably reference or derive from its embedded template, with coverage for the resolution behavior.

Written by the indexing model from the issue text.