feat(site): add favicon (closed green book, old notebook style)

SVG primary (favicon.svg) with PNG fallbacks for browsers/crawlers that
don't support SVG favicons (favicon-32.png) and iOS home-screen icons
(apple-touch-icon.png). Wired into the page shell template and copied
alongside style.css by site-builder.gengo; verify-site.gengo asserts
both the link tags and the built asset files are present.

Author: ganehag

tools/site-builder/site-builder.gengo

@@ -21,6 +21,7 @@ const srcDir    := "root/docs"
 const outDir    := "root/build/site"
 const staticDir := "root/tools/site-builder/static"
 const cssPath   := "style.css"
+const staticAssets := ["favicon.svg", "favicon-32.png", "apple-touch-icon.png"]
 const siteBase  := "https://docs.gengoscript.org/"
 const repoBlob  := "https://github.com/gengoscript/gengo/blob/main/"
 // Keep this in sync with build.zig. The -dev suffix means this site documents
@@ -89,6 +90,9 @@ const pageShellSrc :=
     \\<meta name="description" content="{{.desc}}">
     \\<meta name="robots" content="index,follow,max-image-preview:large">
     \\<meta name="application-name" content="Gengoscript Documentation">
+    \\<link rel="icon" href="favicon.svg" type="image/svg+xml">
+    \\<link rel="icon" href="favicon-32.png" type="image/png" sizes="32x32">
+    \\<link rel="apple-touch-icon" href="apple-touch-icon.png">
     \\<link rel="canonical" href="{{.canonical}}">
     \\<meta property="og:type" content="{{.ogType}}">
     \\<meta property="og:locale" content="en_US">
@@ -980,8 +984,11 @@ func main() {
     sitemap.write("</urlset>\n")
     fs.write(outDir + "/sitemap.xml", sitemap.str())
 
-    // Ship the stylesheet next to the pages.
+    // Ship the stylesheet and favicon assets next to the pages.
     fs.write(outDir + "/" + cssPath, fs.read(staticDir + "/" + cssPath))
+    for name in staticAssets {
+        fs.write(outDir + "/" + name, fs.read(staticDir + "/" + name))
+    }
 }
 
 main()

tools/site-builder/static/apple-touch-icon.png

@@ -34,6 +34,10 @@ assert(fs.exists("root/build/site/index.html"))
 assert(not fs.exists("root/build/site/opcodes.html"))
 assert(fs.exists("root/dev-docs/opcodes.md"))
 
+for name in ["favicon.svg", "favicon-32.png", "apple-touch-icon.png"] {
+    assert(fs.exists("root/build/site/" + name))
+}
+
 // Maintainer-audience audit material must stay contributor-only, not published.
 for name in ["documentation-inventory", "documentation-contradictions", "documentation-plan", "feature-matrix"] {
     assert(not fs.exists("root/build/site/" + name + ".html"))
@@ -52,6 +56,8 @@ for page in fs.list("root/build/site") {
         assert(str.contains(html, "<span class=\"doc-version\">0.5.1-dev"))
         assert(str.contains(html, "<p class=\"doc-status\">Documentation status: main branch / unreleased"))
         assert(str.contains(html, "<meta name=\"robots\" content=\"index,follow,max-image-preview:large\">"))
+        assert(str.contains(html, "<link rel=\"icon\" href=\"favicon.svg\""))
+        assert(str.contains(html, "<link rel=\"apple-touch-icon\" href=\"apple-touch-icon.png\">"))
         assert(str.contains(html, "<meta property=\"og:locale\" content=\"en_US\">"))
         assert(str.contains(html, "<script type=\"application/ld+json\">"))
         assert(str.contains(html, "https://schema.org"))