fix: stabilize snapshots after resize

[cpendery (cpendery)]

Summary

• mark emulator resizes as terminal activity so wait idle starts a fresh quiet period
• wait for asynchronous ConPTY redraws before round-tripping resized snapshots
• extend resize-ordering coverage to prevent the idle-clock regression

Root cause

On Windows, ConPTY can emit a full-screen redraw immediately after the emulator resize returns. The snapshot test could write its baseline before that redraw and compare it afterward, observing two valid row layouts from different frames.

Failure: https://github.com/microsoft/tui-test/actions/runs/32505599633/job/96844938742

Testing

• cargo fmt --all -- --check
• cargo clippy -p tui-test-rs -p tui-test-cli --all-targets --all-features -- -D warnings
• cargo test --workspace -- --test-threads=1
• backend parity regression test passed 30 consecutive runs