feat: crawler finds a page with two H1 headings and stores the issue (tracer)

- golem15_seo_crawls, crawl pages, links, issues and ignores (P33-37)
- classes/crawl: fetchguard client on the site host and its www twin,
  sitemap index and robots.txt seeds, two workers with a delay, limits
- h1_missing and h1_multiple with the P33-38 fix route
- POST /_seo/api/v1/crawls queues the golem15.seo.crawl job; GET /issues
  answers the stored issues with id, severity, source, fix, found_at,
  ignored and crawl_id

classes/crawl/render.go

@@ -0,0 +1,43 @@
+package crawl
+
+import "context"
+
+// Renderer renders a page of the site in a browser for the raw versus
+// rendered comparison (D-70, P33-55): Render answers the page's DOM after
+// JavaScript ran, Close releases the browser. A crawl without a Renderer
+// records the comparison as skipped; a Render error records it as failed
+// with the error and never fails the crawl.
+type Renderer interface {
+	Render(ctx context.Context, pageURL string) (string, error)
+	Close() error
+}
+
+// DefaultRenderMax is the number of pages rendered per crawl when
+// golem15.seo.crawl_render_max is unset.
+const DefaultRenderMax = 200
+
+// The render states of a crawl (models.RenderSkipped and the others).
+const (
+	renderStatusSkipped = "skipped"
+	renderStatusDone    = "done"
+	renderStatusFailed  = "failed"
+)
+
+type rendererKey struct{}
+
+// WithRenderer returns a context whose crawls use r instead of the
+// renderer built from config. It is the test seam of the comparison,
+// settable only from Go code (like fetchguard.WithTransport): no config
+// key, setting, request or MCP call reaches it.
+func WithRenderer(ctx context.Context, r Renderer) context.Context {
+	return context.WithValue(ctx, rendererKey{}, r)
+}
+
+// rendererFrom returns the renderer of ctx, nil when none.
+func rendererFrom(ctx context.Context) Renderer {
+	if ctx == nil {
+		return nil
+	}
+	r, _ := ctx.Value(rendererKey{}).(Renderer)
+	return r
+}