Build the shared build-identity + PWA <head> block.
<head>
Asset path prefix ('' for root pages, '../' for news/ pages).
''
'../'
news/
Multi-line HTML string. Caller is responsible for placing it inside <head>…</head>. The result is already HTML-escaped.
<head>…</head>
Build the shared build-identity + PWA
<head>block.