Let the catalog change while the cluster is running

Adding a model was an operator procedure with downtime: edit a file on the
control plane, restart it, wait out the settling window, watch replicas
churn while adoption catches up. I did that eight times in a day, and one
of the restarts cost two minutes of outage while stalled replicas timed
out. None of it was necessary. The reconciler already compares desired
against actual on every pass, so a new catalog is simply a different
desired.

A Catalog stays immutable — New is the only way to build one and it refuses
an invalid one — and what changes is which immutable catalog is current.
Readers take the pointer and finish their operation against it, so a swap
mid-request is invisible rather than a race. The router is told rather than
asked, the same shape as the endpoint list and for the same reason: a push
happens once per change, a pull would happen once per request.

The router still seeds from configuration as well as being pushed to. A
router with a static endpoint list and no control plane is a supported mode
and has nothing to push it a catalog; without seeding it would resolve no
model at all.

Which turned up a bug that was latent while catalogs were fixed.
reconcileModel only ever visits models that are in the catalog, so a
replica whose model had been removed was visited by nothing: it kept its
GPUs, kept counting, and kept running an engine for a name no client could
ask for. The endpoint publisher skipped such a replica with the comment
"the reconciler will tear it down", which was not true. It is now, and the
comment says what actually happens instead of what was assumed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/control/adminapi/api_test.go

@@ -40,7 +40,7 @@ func setup(t *testing.T, models []catalog.Model, nodes ...inventory.Node) (*API,
 	// anything runs, which is the state settling waits for. Leaving it on would
 	// make every test here assert that placement had not happened yet.
 	cfg.Reconciler.SettleFor = -1
-	ctl := control.New(cfg, cat, nullDispatcher{}, nullSink{}, slog.New(slog.DiscardHandler))
+	ctl := control.New(cfg, catalog.NewLive(cat), nullDispatcher{}, nullSink{}, slog.New(slog.DiscardHandler))
 	for _, n := range nodes {
 		ctl.Inventory().Upsert(n)
 	}