Preview scenes now take paint from the promoted document but GEOMETRY
from the unpromoted layout tree via the design canvas's exact layout
pass — honoring preserve_authored_geometry for Figma imports — so
preview positions match design mode by construction (was: full taffy
re-solve, elements shifted 100+ px on preserve docs, 2 px per promoted
hug widget). Hit-testing maps taps through the deepest painted node's
scene/runtime rect pair with a per-gesture anchor (pointer capture),
so drags never remap through neighbours mid-gesture. Input dispatch
split to preview/input.rs for the 800-line cap.
Pairs with the following device-frame commit (preview/mod.rs already
declares the present module it introduces).
35ce39634b