internal/router/scheduler: add serial scheduler, default on this fork

Add a strict one-model-at-a-time scheduler. Requests run in exact
arrival order; at most one runs at a time; switching to a different
model evicts every other running model first so a single model occupies
memory at a time. Unlike fifo it never reorders or batches same-model
requests, and it ignores group/matrix co-residency entirely, making the
single-model guarantee a property of the scheduler rather than the config.

- new Serial scheduler implementing the Scheduler interface
- register "serial" in scheduler.New; default routing.scheduler.use to
  "serial" at config load (fifo still selectable for upstream behavior)
- update config schema, example config, and config defaults tests

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/config/config_windows_test.go

@@ -266,7 +266,7 @@ groups:
 				},
 			},
 			Scheduler: SchedulerConfig{
-				Use: "fifo",
+				Use: "serial",
 			},
 		},
 	}