visuals: technology marks, area icons, client wall, two diagrams

Phase 7 (AUD-149), cheapest first.

- internal/brand: vendored Simple Icons path data (CC0) for 50 marks, looked up
  by the technology label the content already uses. Inline SVG, 16 px,
  currentColor, no request and no font. tools/gen_brand_icons.sh regenerates it.
- Six hand-drawn area icons replace the two-letter badges in the nav and on the
  landing cards; the evidence index gets a seventh.
- TechChips renders a mark where one exists and plain text where one does not,
  so a missing icon reads as a decision rather than a broken image.
- Client wall: 45 names as one typeface, one size, one grid of equal tiles, and
  the reason for no logo collage written on the page. No mark invented.
- Two diagrams, authored in-repo: homelab topology (platform-network.md) and
  the RAG pipeline (ai-rag-corpus.md), each referenced from the page it explains.
- Content images now carry intrinsic width/height, loading="lazy" and
  decoding="async" at render time; a site test fails the build if an image
  ships without them.
- Topbar is two rows at desktop: the area names plus marks no longer overlap the
  search form (Lighthouse target-size was failing at 1440 px).
- Favicon, so the browser stops requesting a missing /favicon.ico.

Satisfies: logos on every area page, area icons in nav and cards, two diagrams
referenced from their pages, images sized and lazy, Lighthouse >= 95 held.

(cherry picked from commit a473e4738a3751418c2097df2bcf0568fb6d0208)

internal/site/site_test.go

@@ -263,3 +263,35 @@ func TestSearchIndexFollowsContent(t *testing.T) {
 		t.Errorf("token found at %s, want /edge/edge-fleet/", results[0].URL)
 	}
 }
+
+// TestImagesReserveTheirSpace is the layout-shift guard: every image a content
+// page carries must declare its intrinsic size and load lazily, so a page of
+// diagrams and screenshots never reflows as the files arrive.
+func TestImagesReserveTheirSpace(t *testing.T) {
+	s := newSite(t)
+	seen := 0
+	for _, d := range s.Content.Docs {
+		html := d.HTML
+		for i := 0; ; i++ {
+			start := strings.Index(html, "<img ")
+			if start < 0 {
+				break
+			}
+			end := strings.Index(html[start:], ">")
+			if end < 0 {
+				break
+			}
+			tag := html[start : start+end+1]
+			html = html[start+end+1:]
+			seen++
+			for _, want := range []string{`width="`, `height="`, `loading="lazy"`, `alt="`} {
+				if !strings.Contains(tag, want) {
+					t.Errorf("%s: image tag missing %s: %s", d.Path, want, tag)
+				}
+			}
+		}
+	}
+	if seen == 0 {
+		t.Fatal("no images found in content; the guard is not exercising anything")
+	}
+}