feat(17-03): sitemap plugin tracer, /sitemap.xml byte-identical to PHP

- golem15.sitemap plugin: golem15_sitemap_definitions migration, Definition
  model (Fillable theme and data) with tolerant PHP-shaped item decoding and
  PHP toArray item encoding
- Url::to and buildUrl port, libxml-format serializer, generator for url and
  registry items
- GET /sitemap.xml serves the definition with theme default (404 without one,
  empty body without items)
- tests/php/gen-goldens.php and the tracer golden; TestSitemapGolden and the
  Postgres TestSitemapTracer

plugin.go

@@ -0,0 +1,50 @@
+// Package sitemap is the golem15.sitemap plugin: it serves /sitemap.xml
+// from the application's one sitemap definition, byte for byte the way the
+// WinterCMS Golem15.Sitemap plugin did, with items that are explicit URLs
+// or menu item types of the framework's registry.
+package sitemap
+
+import (
+	"git.golem15.com/golem15/sm-sitemap-plugin/models"
+	"git.golem15.com/golem15/sm-sitemap-plugin/updates"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/pact"
+	"git.golem15.com/golem15/summercms/modules/party"
+	"github.com/go-gormigrate/gormigrate/v2"
+)
+
+var (
+	_ party.Plugin       = (*Plugin)(nil)
+	_ pact.HasMigrations = (*Plugin)(nil)
+	_ pact.HasModels     = (*Plugin)(nil)
+	_ pact.HasRoutes     = (*Plugin)(nil)
+)
+
+// Plugin is the golem15.sitemap plugin.
+type Plugin struct {
+	app *backpack.App
+}
+
+func (p *Plugin) ID() string { return "golem15.sitemap" }
+
+// Requires is empty: the sitemap reads content types through the framework's
+// menu item type registry, not from a named plugin.
+func (p *Plugin) Requires() []string { return nil }
+
+func (p *Plugin) Register(app *backpack.App) error {
+	p.app = app
+	return nil
+}
+
+func (p *Plugin) Boot(app *backpack.App) error {
+	p.app = app
+	return nil
+}
+
+func (p *Plugin) Migrations() []*gormigrate.Migration { return updates.All() }
+
+func (p *Plugin) Models() []any { return models.All() }
+
+func init() {
+	party.Register(&Plugin{})
+}