The URL says which provider set runs, and the seam stops claiming otherwise

The seam arrived carrying a switching design that no longer holds. It presented
runtime hot-switching as what shaped it — selection in state, a key on <Refine>
to drop the previous backend's cache — and it read the backend from a build
global whose comment claimed build.ts defined it. Nothing ever did, and nothing
could have: this project serves from src/ in production too, so that define
block never reaches what runs.

What replaces it is a URL. The default provider set lives at the root, another
behind a path prefix naming it, and moving between them is a link. The reload is
the mechanism rather than a cost to avoid: it discards the cache and the session
with no code to undo them. So the switch helpers are gone and the file is
smaller than what it replaced.

The resolution is a pure function with its own tests, including the case that is
true today — an empty registry, where every URL is the default. The router does
NOT yet strip the prefix, deliberately: it is a shared primitive every screen
depends on, the suites that would prove a change to it are not run here, and no
URL carries a prefix while one backend is registered. That half lands with the
second backend, which is what makes it testable.

Three claims the move to this base falsified are corrected too. The inbox the
count comment points at is local here, not on another branch. The worked
provider implementation is not in this branch's ancestry. And the MeetingPoint
shape exists here, so its binding stops being optional — a backend that cannot
carry one of the four entities cannot serve the app, and nothing optional should
let a half-backend register quietly.

Also: both usage declarations are named usage_festipod.md, so the stem is now
ambiguous as a link. They are referred to by folder path, and both are listed
under publish so each provider can re-find its own if it moves.

.project/contracts.yaml

@@ -23,8 +23,11 @@
 # committed.
 
 publish:
-  # Key qualified by interface: a second declaration is coming for the NextGraph+polyfill
-  # backend, and publish keys must stay unique.
+  # Both declarations are named `usage_festipod.md`, so the keys are qualified by their
+  # interface — publish keys must stay unique, and the bare subject no longer says which.
+  # Listing a declaration here is what lets its provider re-find it with `list --from`
+  # after it moves; it sends nothing on its own.
+  polyfill-surface--festipod: concepts/data-layer/polyfill-surface/usage_festipod.md
   activitypods-providers--festipod: concepts/data-layer/activitypods-providers/usage_festipod.md
 
 consume: