feat(frontdoor): route provider-qualified model names on every session

[Mearman]

feat(frontdoor): route provider-qualified model names on every session

A qualified model name was only resolved on a session that asked to
route by name, so exposing another provider's models meant a launch flag
or a router provider. The rule is now the naming scheme itself: a model
name whose first slash-separated segment names a defined provider routes
there on every session, with the request rewritten under the target's own
model name, and a slash naming no provider (an aggregator's vendor/model)
stays the session's own provider's business. A name for the provider the
session is already on is served in place under its bare model name.

Bare names keep their opt-in: only a session that routes by name (a
router's included) resolves a virtual model's or a family's name, so
nothing a session types bare is ever hijacked, and --route-by-name
now means exactly that.

/model follows: every session lists every provider's models qualified
(its own provider's catalogue included), a provider session's rows
replacing the built-in lineup and a provider-less session's following it;
the virtual and family rows stay on the sessions that can serve them. The
provider list the rows are built from is read through the launch's own
filesystem port, so an invalid provider file lists no rows rather than
failing the launch.

One scan now runs for every routed request, so the name resolution's
replay is carried into the routes matching that follows it; reading the
original body twice left two consumers on one stream and hung the
request.