fix(agent): wait for snapshot frames on DOM events, not a 16ms timer poll

[ngocht]

Summary

waitForAgentRenderFrame polled every 16ms with chained setTimeouts. In a background tab Chrome throttles chained timers to once per second, and to once per minute after five minutes hidden. An MCP site_render_snapshot routed to an unfocused editor could therefore stall for minutes.

It now re-checks on a MutationObserver over the canvas DOM and the readiness markers (data-instatic-canvas-document-loaded, data-agent-snapshot-ready, the breakpoint and request-id attributes). Observer callbacks are microtasks and are not throttled. A 250ms timer stays as a fallback; the 5s deadline is unchanged.

Impact: render snapshots resolve as soon as the frame is ready, whether or not the editor tab is focused.

Verification

• bun run build — ran tsc -p tsconfig.app.json (clean)
• bun test — ran bun test src/__tests__/agent/: 269 pass. New renderFrameWait.test.ts simulates throttled timers (every timer ≥1s): resolves in <500ms with this change, fails (1035ms) on main.
• bun run lint — eslint on the changed files (clean)
• Docker/deployment check, if relevant

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed.
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.

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