Navigate authenticated actors before multi-actor browser actions

[chubes4]

Problem

The merged wordpress.browser-scenario multi-actor runner creates isolated contexts, installs each actor authentication cookies, and immediately dispatches scenario actions, but it never navigates actor pages to scenario.url. Evaluate and interaction actions therefore execute on about:blank; the current example only uses duration waits and does not detect this.

This blocks the 12-distinct-author downstream proof for WordPress/gutenberg#79020.

Expected contract

After every actor context is provisioned and capture listeners are attached, navigate all actor pages concurrently to the resolved scenario URL and wait for load before dispatching the seeded action schedule. Preserve existing request-gate instrumentation and per-actor network evidence.

Acceptance criteria

• A deterministic runner test proves every actor starts at the resolved scenario.url, not about:blank.
• Actor navigation happens after auth cookies and capture listeners are installed.
• Actor pages navigate concurrently before scheduled actions begin.
• Navigation failures identify the affected actor and retain the existing partial artifact evidence.
• Existing multi-actor scheduling, barrier, and gate tests remain green.

Follow-up to #1899 and #1913.