Narrowly recovers slot metadata for legacy wire-v1/v2 router plugins that
export route. Restricted to the route -> Router migration so we do
not bypass the handshake’s implemented_functions source of truth for
other slots; new uploads and shape / observe plugins must come through
the handshake gate.
Returns the slots whose wire function the plugin claims to implement.
The result is deterministically sorted, deduplicated, and ignores unknown
function names so newer plugins remain forward-compatible with this host.