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

classes/testdata/tracer.json

@@ -0,0 +1,18 @@
+{
+  "case": {
+    "root": "https://api.example.test",
+    "items": [
+      {"type": "url", "url": "/", "changefreq": "always", "priority": "1.0", "nesting": "0"},
+      {"type": "acme-post", "reference": "7", "urlPattern": "", "changefreq": "weekly", "priority": "0.5", "nesting": "1"}
+    ],
+    "types": {
+      "acme-post": {
+        "7": {
+          "url": "/news/hello",
+          "mtime": "2026-03-04T10:00:00+00:00",
+          "alternates": [["pl", "/news/hello"], ["en", "/en/news/hello"]]
+        }
+      }
+    }
+  }
+}