Make controlled transition frame-demand gate deterministic #88

Problem

The packaged native-runtime matrix intermittently fails webscene_native_engine_tests at the controlled-switch transition gate even though the click, controlled-property semantics, event order, focus state, and computed transform have already passed:

webscene_native_engine_tests: controlled switch transform transition did not request a host frame

Observed on the Linux x64 package job for consolidated Code OSS compatibility commit 2e86f1ed; the same unchanged gate passed immediately before and on the hosted CI/native document jobs. The test reads webscene_engine_requires_animation_frame() once after work crosses the engine thread, while the suite already treats frame-demand publication as an asynchronously observable state elsewhere.

Proposed fix

Use the existing bounded wait_for_animation_frame_demand(engine, true, ...) test helper. Keep the 500 ms deadline and diagnostic demand value. Do not add an unconditional sleep, reduce transition assertions, or retry the whole CI job.

Acceptance criteria

• The controlled switch still asserts native click semantics, event order, focus/focus-within, and the initial transform.
• Frame demand is observed through the existing bounded polling contract.
• The full native engine test remains deterministic in packaged Linux x64, macOS arm64, and Windows x64 runs.
• No production scheduling behavior or performance budget is weakened.

The fix is included on draft PR #76 so it is qualified with all three packaged runtimes before merge.