[Mearman]

feat(frontdoor): resolve every model name through the models block, and drop route-by-name

The route-by-name opt-in is gone. A bare model name is now resolved
through the models block on every session, with one guard: a name the
session's own provider serves itself stays that provider's, because the
launch chose it for its own names, and a name only the block defines is
the block's to serve. A name neither defines goes where it was sent.
Qualified names keep the rule the previous release gave them.

The launch flag, its environment variable, the profile key and the door's
session header are removed rather than left meaning nothing: a config file
still carrying launch.routeByName is warned about and ignored like any
other setting this release does not know. The front-door protocol bumps to
5 so a launcher without the header replaces a door that still gates on it.

Two behaviours the flag used to gate become unconditional: the tier
pointers a models block defines are applied on every launch, and the
credential preflight across the block's providers runs whenever a block
exists, so a provider whose token yields nothing still fails the launch
naming it.