Add fixed browser preview double buffering

[chubes4]

Summary

• add a product-neutral two-slot browser preview buffer with explicit prepare, activate, navigate, release, and dispose operations
• keep the active iframe visible while a standby Playground runtime starts and make activation atomic
• normalize direct local ability materialization results without installing the provider proxy
• remove released lifecycle replacement iframes and retain bounded, idempotent release receipts

Follow-up to #2175.

Verification

• npm run test:browser-sdk-facade
• npm run test:browser-runner-template
• npm run test:service-worker-preview-proxy

The packaged Playground browser test proves two live slots, atomic activation, explicit runtime replacement, standby replenishment, and bounded teardown.

Remaining Acceptance Work

Downstream end-to-end switching renders the first site, a second site, and the first site again with isolated markers. A repeatable 60-second timeout can still occur while starting the initial second blank Playground runtime, before teardown or navigation. This PR remains draft while that runtime-capacity blocker is isolated; no private evidence URLs or local paths are included here.

AI Assistance

OpenAI gpt-5.6-sol via OpenCode assisted with implementation, regression analysis, test execution, and PR preparation. Chris Huber directed the architecture and reviewed the resulting changes.