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/content/content.go

@@ -37,22 +37,21 @@ type Area struct {
 	Title  string
 	Role   string
 	Blurb  string
-	Gutter string // two-letter mark used by the nav and the stage panel
 }
 
 // Areas is the fixed navigation order from reference/content-map.md.
 var Areas = []Area{
-	{Key: "ai", Title: "AI & Agentic Systems", Role: "AI / LLM Engineer", Gutter: "AI",
+	{Key: "ai", Title: "AI & Agentic Systems", Role: "AI / LLM Engineer",
 		Blurb: "Harnesses, subagents and skills, memory, model routing and cost, retrieval, voice, guardrails, agent observability."},
-	{Key: "platform", Title: "Platform & Infrastructure", Role: "Platform / DevOps Engineer", Gutter: "PL",
+	{Key: "platform", Title: "Platform & Infrastructure", Role: "Platform / DevOps Engineer",
 		Blurb: "Declarative hosts, packaging, a fifty-container Docker estate, networking, storage and restore-tested backups, observability."},
-	{Key: "edge", Title: "Edge & Devices", Role: "Edge / IoT Engineer", Gutter: "ED",
+	{Key: "edge", Title: "Edge & Devices", Role: "Edge / IoT Engineer",
 		Blurb: "Twenty-node fleet on constrained devices, custom radio, solar power, deep sleep, sensors and actuation — and the failures."},
-	{Key: "products", Title: "Products", Role: "", Gutter: "PR",
+	{Key: "products", Title: "Products", Role: "",
 		Blurb: "Three active businesses, built and run rather than advised on."},
-	{Key: "history", Title: "History", Role: "", Gutter: "HI",
+	{Key: "history", Title: "History", Role: "",
 		Blurb: "2000–2014: agencies, clients, engagements, and the code that survives from them."},
-	{Key: "about", Title: "About", Role: "", Gutter: "AB",
+	{Key: "about", Title: "About", Role: "",
 		Blurb: "The narrative, the non-IT work that came first, education, and how to get in touch."},
 }