PsHandler iterated sched.loaded without holding loadedMu, racing with
scheduler goroutines that mutate the map. It also read runnerRef fields
(model, llama, expiresAt) that unload() and the expiration path mutate
under refMu, so a concurrently unloading runner could nil model out from
under the handler.
Instead of adding locking in routes.go, give the scheduler a small
snapshot API: loadedModels() copies the runner list under loadedMu, then
captures each runner's reporting fields under its refMu, respecting the
refMu-before-loadedMu lock ordering used by the expiration path. The
zero-expiresAt estimate for still-loading models moves into the
scheduler too, since it exists because of scheduler behavior.
Also remove the dead code Scheduler.GetRunner
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