Optimize frozen capture export performance

[yvette-carlisle]

Summary

• move frozen capture copy/save rendering and PNG generation off the main thread
• prepare reusable frozen export PNGs after freeze commit so copy/save can hit cache
• keep direct RGBA overlay export fast path and fix export Y-axis mapping for annotations

Verification

• cargo test -p rsnap-overlay frozen_export -- --nocapture
• cargo make fmt-check
• cargo make check-vstyle
• cargo make check-rust
• cargo make check-swift
• cargo make test-macos-native-host
• git diff --check
• ./scripts/build_and_run.sh --verify