Flush parser insertion reactions after the tree builder completes its current
token so connected callbacks run before children without retaining parser
borrows across JavaScript execution.
Keep one DocumentParserSession and expose a restricted insertion handle for
nested writes. Share input, suspension, cancellation, and close state so
blocked writes remain queued and outer feeds stop when nested parsing blocks
or replaces the document. Let the owning session finish deferred close and
dispatch readiness transitions once.
Reuse form-owner mutation detection for child parser insertions and add
regressions for reaction order, form association, nested writes, gated external
scripts, document replacement, and deferred close.
Validation: cargo fmt --all; strict workspace Clippy; cargo nextest run
--no-fail-fast (18,104 passed, 13 existing skips); release build.
ldm0