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/config/config_schema_test.go

@@ -0,0 +1,60 @@
+package config
+
+import (
+	"encoding/json"
+	"os"
+	"testing"
+
+	"github.com/google/jsonschema-go/jsonschema"
+	"gopkg.in/yaml.v3"
+)
+
+// TestConfig_ExampleMatchesSchema validates that config.example.yaml conforms to
+// config-schema.json. Both files live at the repository root.
+func TestConfig_ExampleMatchesSchema(t *testing.T) {
+	const (
+		schemaPath  = "../../config-schema.json"
+		examplePath = "../../config.example.yaml"
+	)
+
+	schemaBytes, err := os.ReadFile(schemaPath)
+	if err != nil {
+		t.Fatalf("reading %s: %v", schemaPath, err)
+	}
+
+	var schema jsonschema.Schema
+	if err := json.Unmarshal(schemaBytes, &schema); err != nil {
+		t.Fatalf("unmarshalling schema: %v", err)
+	}
+
+	resolved, err := schema.Resolve(&jsonschema.ResolveOptions{
+		BaseURI: "https://github.com/mostlygeek/llama-swap/",
+	})
+	if err != nil {
+		t.Fatalf("resolving schema: %v", err)
+	}
+
+	exampleBytes, err := os.ReadFile(examplePath)
+	if err != nil {
+		t.Fatalf("reading %s: %v", examplePath, err)
+	}
+
+	// Convert YAML to a JSON-like value so numbers and keys match what the
+	// validator expects.
+	var yamlValue any
+	if err := yaml.Unmarshal(exampleBytes, &yamlValue); err != nil {
+		t.Fatalf("unmarshalling example yaml: %v", err)
+	}
+	jsonBytes, err := json.Marshal(yamlValue)
+	if err != nil {
+		t.Fatalf("converting example to json: %v", err)
+	}
+	var instance any
+	if err := json.Unmarshal(jsonBytes, &instance); err != nil {
+		t.Fatalf("unmarshalling example json: %v", err)
+	}
+
+	if err := resolved.Validate(instance); err != nil {
+		t.Fatalf("config.example.yaml does not match config-schema.json:\n%v", err)
+	}
+}