Add coordinated multi-actor browser scenarios

[chubes4]

Problem

WP Codebox recipes can provision multiple fixture users and persisted user sessions, but browser execution is serial and each browser command owns a single Playwright context/page. This prevents deterministic testing of concurrent authenticated workflows such as overlapping edits and saves.

Downstream workloads currently have to invent application-specific orchestration, which loses reusable scheduling, replay, and artifact guarantees. Gutenberg PR WordPress/gutenberg#79020 is the immediate motivating case.

Proposed contract

Add a generic browser-scenario primitive that:

• declares named actors bound explicitly to recipe user sessions
• keeps an isolated browser context/page per actor
• executes a seeded, deterministic schedule
• supports named barriers for coordinated progress
• supports bounded request gates for delay, release, retry, and out-of-order response schedules
• records actor actions, barrier transitions, request timing/order, seed, and replay inputs
• emits per-actor screenshots, traces, console/network evidence, and final scenario state
• tears down all contexts and gates deterministically on success or failure

Acceptance criteria

1. A recipe can run at least two authenticated actors against one WordPress runtime.
2. A deterministic test proves both actors can pause at a barrier and resume in the declared order.
3. A deterministic test proves one actor request can be held and released after another actor action.
4. Failure and timeout paths identify the actor/barrier/gate and still persist replayable artifacts.
5. Existing single-browser recipe behavior remains unchanged.
6. The public schema, runtime types, validation, examples, and focused tests describe the contract without Gutenberg-specific concepts.

Downstream proof

Use the primitive to model concurrent note authors, stale saves, retries, and convergence for WordPress/gutenberg#79020.