Add coordinated multi-actor browser scenarios

[chubes4]

Summary

Adds reusable seeded multi-actor Playwright scenarios to wordpress.browser-scenario with isolated authenticated sessions and replayable evidence.

What changed

• Adds isolated actor contexts, seeded action scheduling, barriers, occurrence-aware request gates, deterministic cancellation/teardown, and actor-qualified screenshots, traces, logs, replay, request coverage, and waterfall artifacts while preserving the existing single-browser path.

How to test

1. Run npm ci; expect Dependencies install successfully..
2. Run npm run build; expect TypeScript packages compile successfully..
3. Run npm run test:browser-scenarios; expect Multi-actor browser scenario tests pass..
4. Run npm run test:recipe-user-session; expect Recipe user-session tests pass..
5. Run npm run test:schema-parity; expect Schema parity tests pass..
6. Run npm run test:public-api-contract; expect Public API contract tests pass..

Compatibility

Additive contract change: request-gate occurrence is optional, and recipes without actors continue through the existing single-browser implementation unchanged.

Evidence

• Base unchanged since verification: main remains at 13ccafb.
• Durable run execution: Succeeded
• Reviewer-resolvable source evidence: Add coordinated multi-actor browser scenarios #1899
• Verified finalization base: main at 13ccafb
• gate-1: Passed
• gate-2: Passed
• gate-3: Passed
• gate-4: Passed
• gate-5: Passed

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode via Homeboy
• Model: openai/gpt-5.6-terra
• Used for: Implemented the runtime, tests, documentation, provider refinement, and deterministic verification; Chris reviewed and owns the change.

Source relationships

• Closes Add coordinated multi-actor browser scenarios #1899