Mark engine as done in overview and build order. Update next-steps to
focus on test gap analysis. Add engine file map, data flow, and dev
setup quirks (Zeitwerk conflict, engine routes conflict) to architecture.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Overview, architecture notes, and next steps for anyone
picking this up — including Rails API gotchas discovered
during implementation.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
