Keep blocked writes with the input position saved by each parser script. Share the context between main and child parsers so external-script suspension preserves nested caller tails and consecutive write order.
Add four integration regressions across main/child and navigation/script-created parsers. Validated with fmt, strict workspace Clippy, 19474 passing nextest tests, 18 Chromium comparison checks, and 50 WPT pages without new failures.
