feat: use frontend-base compatibility layer

[arbrandes]

Description

Lets a frontend-base site keep working with FPF-built plugins as a migration aid during the MFE deprecation window. The shim is opt-in: plugin authors register an (mfe_name, slot_name, plugin_config) triple in FRONTEND_COMPAT_SLOTS to route a single slot through the compat layer, or list a Tutor plugin name in FRONTEND_COMPAT_PLUGINS to bulk-opt every contribution that plugin makes via PLUGIN_SLOTS. Sites with no opted-in contributions are unaffected.

Requires tutor@main: depends on the unreleased ENV_SAVED action.

Implementation Details

Renders one env.config.<mfe_name>.jsx per MFE alongside the legacy per-MFE env.config.jsx, each carrying just the contributions opted in for that MFE. site.config.{build,dev}.tsx registers each one as its own shim App via createLegacyPluginApp, passing the MFE's name as mfeId so the shim can route-scope its contributions. The wildcard "all" is itself a valid mfe_name: it renders env.config.all.jsx and mounts it without an mfeId, so its contributions fire globally across every route, mirroring the legacy iter_plugin_slots("all") semantics. site/package.json adds the compat dep and an npm overrides block redirecting both @openedx/frontend-plugin-framework and @edx/frontend-platform to the same compat package, so transitive imports resolve to the shim's stubs and neither real package is installed.

Two new filters drive opt-in. FRONTEND_COMPAT_SLOTS is the per-slot opt-in registry, mirroring the (mfe_name, slot_name, plugin_config) shape of PLUGIN_SLOTS. FRONTEND_COMPAT_PLUGINS is a coarser bulk opt-in by Tutor plugin name, folding every PLUGIN_SLOTS contribution from that plugin's hook context through the shim.

Three more filters layer (legacy_id, mapping) deltas onto the shim's curated defaultSlotMap, defaultWidgetMap, and defaultRouteMap for ids the shim doesn't yet cover: FRONTEND_SLOT_COMPAT_MAPS, FRONTEND_WIDGET_COMPAT_MAPS, and FRONTEND_ROUTE_COMPAT_MAPS. These deltas are site-wide (not per-MFE), so site.config.{build,dev}.tsx materializes the merged maps once and passes them by reference into every createLegacyPluginApp call. Identical contributions dedup silently; divergent ones warn and last-wins.

All compat wiring is gated on get_frontend_compat_mfes() being non-empty, and tagged TODO(fpf-removal) for cleanup on the FPF deprecation timeline. Full design captured in openedx/frontend-base-compat ADR 0001.

Testing

The easiest way to test this is with tutor-indigo, via this second PR:

overhangio/tutor-indigo#219

After tutor config save and tutor images build mfe, you should see the indigoified header in the new Instructor Dashboard app:

LLM usage notice

Built with assistance from Claude.

[arbrandes]

We'll have to remember to remove this once it lands in Verawood. But without it, CI will fail.