feat: align code-demo nav with midi-demo nav style

Replaces the flat tag-row header in code-demo.html with the same three-zone layout used in midi-demo.html: - Left: 'Muse' logo (monospace, accent colour) - Centre: title 'Hexagon API Gateway · Muse VCS' + metadata row (6 acts · 6 commits · 4 branches · 25 symbols, module tags, conflict/agent/language badges) - Right: v0.1.2 version badge + MIDI Demo / Code Demo links

Nav height bumped 52px → 60px and body padding-top updated to match.