Existing EditorCommand variants are leaf-only (BatchInsertItem carries
only kind/name/x/y/w/h/fill_hex). The design orchestrator (S3a) must
apply rich nested designs — frames with children, layout, text — so
add InsertSubtree { nodes: Vec<PenNode>, parent_id }.
cmd_insert_subtree validates the parent is a container (or NONE = page
root), remaps every incoming node id to a fresh editor id via
remap_subtree_ids (so an externally-authored subtree can't collide
with live ids), and appends under the parent. The apply arm wraps it
in a history snapshot so the insert is one undo step.
NOT verified locally: op-editor-core does not currently build —
vendor/jian is pinned to unpushed commit 80121906 whose DesignMd*
types op-editor-core depends on are absent from every available jian.
The 8 InsertSubtree tests in command_subtree_tests.rs run once the
jian build is restored.
S3a Plan A.
b1c3136deb