A paragraph with keep with next measures the paragraph after it before it is
placed (next_first_line). That measurement called layout_para directly, so
every paragraph following a heading was laid out twice in each layout pass:
once to measure it and once, through the cache, when it was placed.
The measurement now goes through the layout cache under the key
para_labelled uses (Ctx::para_ahead) when its inputs are the same: no
proofing, no typed word, not inside a text box, no inline text boxes that
resize, the body's tracked-deletion view. Otherwise it is laid out on its
own as before. It counts nothing (list labels, equation numbers and the
page-total flag are restored), so pagination is unchanged.
Adds crates/engine/examples/export_bench.rs, which saves a long generated
document (the sample document 200 times, 259 pages) through io::save_bytes
and prints wall times:
cargo run --release -p wordcraft-engine --example export_bench -- pdf 200 9
PDF export on that document, interleaved with main (Ryzen, 16 threads):
median 286 ms -> 256 ms (-10.6 %). The PDF bytes are identical.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
