ReadonlypathThe scene-graph path this type's node lives at in makeCommitScene.
A node of this type, shaped the way its node loader leaves it before the first commit.
Commit count elements to the node at path. With extending, the
payload carries prefix lineage to the node's last commit — the proof a
progressive ladder's next rung carries that it EXTENDS what is drawn
(types/prefix-lineage.ts), which the instanced commits' append path keys on.
Mesh ignores extending: its loader stamps no lineage and its commit
rewrites the whole index buffer.
Optionalextending: boolean
Per-type node placeholder + commit, both through real code.