Merge Kayshen's feat/responsive-m1a (schema 1.2 + responsive opt-in,
breakpoint ranges, variant tables, transactional variant swap) plus
main's fill-rule/corner-radii/background-blur into the vendored pin
(merge/responsive-m1a-into-main @ 42772b6). Responsive stays double-
gated opt-in, so every existing document renders unchanged.
Adaptation is mostly new optional fields defaulted to None/false. The
one real behavior interaction: jian's node_rect now bakes a root's
authored origin into its returned rect (root_origins, added for the
responsive multi-root canvas), which doubled offsets at the three
places that added the origin themselves — the loader's layout harvest,
App Mode tap solving, and the preview's scene-to-runtime-space mapping
(the last now subtracts the root origin back out, gated on jian's own
is_origin_normalized so responsive roots stay untouched).
