Fix preview/browser proof reliability

[peter-olom]

What changed

• Bypass the runtime broker JSON parser for /routes/preview/* so preview POST/API request bodies stream through to the preview app instead of being consumed before proxying.
• Add short retries around broker-to-Playwright browser proof control calls and replace raw transient fetch failed errors with an actionable readiness message.
• Add focused regression coverage for preview POST proxy body handling and Playwright proof sidecar retry messaging.

Tests and build checks

• node --check docker/runtime-broker/broker.mjs
• node --check docker/runtime-broker/broker-browser.mjs
• node --test docker/runtime-broker/broker-browser-annotation.test.mjs
• In disposable snapshot preview: cd butler && npm ci
• In disposable snapshot preview: cd butler && npm test -- test/server/runtime-broker-preview-isolation.test.ts && cd .. && node --test docker/runtime-broker/broker-browser-annotation.test.mjs (executed the Butler TS suite plus Docker browser regression)
• In disposable snapshot preview: cd butler && npm run build

Proof / deployment note

• Attempted manor-harness preview use start against a ready disposable preview on the current live stack; it still returned raw fetch failed, confirming the source blocker remains live until Manor is rebuilt/restarted with this PR. No live restart or deploy was performed.

Remaining risk

• Requires rebuilding/restarting the Manor runtime-broker/playwright services for the broker changes to affect live proof capture.

[peter-olom]

Closing as superseded/stale: this work has been replaced by later merged Manor work.