Carry RequestHeaders through navigation, Fetch/XHR, workers, redirects and auth,
with explicit Unicode and ByteString conversions at protocol and WebIDL boundaries.
Share the raw curl header list with WebSocket transport and decode binary response
headers without UTF-8 replacement. Keep header memory charging concrete.
Cover opaque values, UTF-8 overrides, duplicates, redirect/auth continuation and
WebSocket handshakes with byte-level regression tests.