Implement new scheduler (#823)

- introduce internal/router/scheduler to decouple routing, swapping and
queuing into interface contracts.
- introduce a new `routing` configuration section that supersedes
`matrix` and `group` while maintaining backwards compatibility
- add FIFO scheduler with prioritized queuing 
- add internal/router/design.md as developer documentation on
implementing new schedulers and routers

Fixes #797

internal/router/helpers_test.go

@@ -12,10 +12,23 @@ import (
 	"testing"
 	"time"
 
+	"github.com/mostlygeek/llama-swap/internal/config"
 	"github.com/mostlygeek/llama-swap/internal/logmon"
 	"github.com/mostlygeek/llama-swap/internal/process"
 )
 
+// groupRouting builds a normalized RoutingConfig for the group router, mirroring
+// what config.LoadConfigFromReader produces. Tests use it to populate
+// config.Config.Routing without going through LoadConfig.
+func groupRouting(groups map[string]config.GroupConfig) config.RoutingConfig {
+	return config.RoutingConfig{
+		Router: config.RouterConfig{
+			Use:      "group",
+			Settings: config.RouterSettings{Groups: groups},
+		},
+	}
+}
+
 // fakeProcess is an in-memory implementation of process.Process used to drive
 // the routers through their state machine without spawning real upstreams.
 type fakeProcess struct {