feat(11.1-05): add the acme.blog walkthrough plugin with its model, migration and posts route

- docs/examples/blog: scaffolder output for acme.blog (make:plugin, make:model)
  in the root module, with a Post model, a fill allow-list and NewPost, the
  create migration and GET /api/blog/posts paginated through lagoon
- short tests activate the plugin, check the route with surf, the fill
  allow-list and the migration order
- docs/setup/porting-a-plugin.md: registration, model, migrations and routes
  sections with src= copies of the plugin
- TestDocsRequiredPages requires setup/porting-a-plugin

docs/examples/blog/updates/updates_test.go

@@ -0,0 +1,57 @@
+package updates_test
+
+import (
+	"os"
+	"regexp"
+	"slices"
+	"strings"
+	"testing"
+
+	"git.golem15.com/golem15/summercms/docs/examples/blog"
+	"git.golem15.com/golem15/summercms/docs/examples/blog/updates"
+)
+
+var migrationFile = regexp.MustCompile(`^(\d{14})_[a-z0-9_]+\.go$`)
+
+func TestMigrationIDs(t *testing.T) {
+	if got := updates.CreatePosts().ID; got != "20260101000000_create_acme_blog_posts" {
+		t.Errorf("CreatePosts ID = %q", got)
+	}
+
+	// The plugin returns the set in registry.gen.go's order, which must be
+	// ascending by ID: gormigrate applies it in slice order.
+	var ids []string
+	for _, m := range (&blog.Plugin{}).Migrations() {
+		if m.Migrate == nil || m.Rollback == nil {
+			t.Errorf("migration %s has no Migrate or Rollback", m.ID)
+		}
+		ids = append(ids, m.ID)
+	}
+	if len(ids) == 0 {
+		t.Fatal("the plugin has no migrations")
+	}
+	if !slices.IsSorted(ids) {
+		t.Errorf("migration IDs are not in ascending order: %v", ids)
+	}
+
+	// Every migration file is named after its ID, and every ID has a file.
+	entries, err := os.ReadDir(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var files []string
+	for _, e := range entries {
+		name := e.Name()
+		if e.IsDir() || name == "doc.go" || strings.HasSuffix(name, "_test.go") || !strings.HasSuffix(name, ".go") {
+			continue
+		}
+		if !migrationFile.MatchString(name) {
+			t.Errorf("%s does not start with a 14-digit timestamp", name)
+			continue
+		}
+		files = append(files, strings.TrimSuffix(name, ".go"))
+	}
+	if !slices.Equal(files, ids) {
+		t.Errorf("migration files %v, want one per ID %v", files, ids)
+	}
+}