feat(engine): offer a box layout before best-fit pairs in Fill

Interactive Fill ran the best-fit pair phase first. The first pair
computation for a drawing can take about a minute, and until it finished
the progress dialog showed no layout, Accept stayed disabled and the
status text did not change.

The fill now packs the part boxes (RectBestFit) before the pair phase and
reports that layout, so Accept is available at once. The layout is only a
fallback: strategies still run in their usual order, and a completed fill
picks the same winner as before. While the pairs are computed, the detail
line reads "Computing best-fit pairs (may take a minute)...". Accept or
Stop now stops the wait. The computation keeps running in the background
and is cached for the next fill.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/nesting-engines.md

@@ -30,6 +30,15 @@ Rectangles places irregular parts validly, but only as their bounding boxes; it
 notch or hole. Box sides account for how the layout check flattens arcs, so round-edged parts stay
 valid at box contact.
 
+Interactive Fill shows its rectangle best-fit (box packing) layout before the best-fit pair
+phase, whose first computation for a drawing can take a minute, so **Accept** is available at
+once. The box layout is only a fallback: the strategies still run in their usual order and a
+completed fill picks the same winner as without it. While the pairs are computed the progress
+detail reads "Computing best-fit pairs (may take a minute)...". Accepting or stopping stops waiting for the
+pair computation, which finishes in the background and is cached for the next fill. Fills
+inside clamp-free areas of a clamped plate report only their final certified proposal, so this
+early layout is not shown there.
+
 Interactive/full-area box packing uses the same 90% work-area slack allowance as Rectangles.
 A free box may absorb a slightly oversized side only at its right/top edge when that edge
 coincides with the plate work-area boundary. Internal leftover edges keep the strict packing