v2: Authentication helpers

[loyality7]

Context

Convenience over what v1 already allows (headers, cookies) — not new capability. Reduces the boilerplate every caller doing authenticated automation currently repeats by hand.

Scope

• Basic auth credential helper.
• Header injection helper (wraps existing setExtraHeaders once that's real, see the tracking note below).
• Cookie import helper (wraps existing setCookie).

Dependencies

setExtraHeaders is currently a documented no-op stub in PageImpl — this item needs that wired for real first, or needs to wire it as part of this work.

Acceptance criteria

• Basic auth credentials can be supplied and a protected fixture page loads without a browser-native prompt
• Header injection actually reaches every request, not just the top-level navigation
• Cookie import helper is a thin, tested wrapper — no new underlying mechanism