* fix(fig): keep Figma's order keys on instances and untouched siblings
Materialized instance occurrences were built without the record's
parentIndex position, so every imported instance lost its key and was
renumbered on save. They now carry it like other imported nodes.
Sibling keys anchored greedily on the first imported key, so a layer moved
up past its siblings kept its key and pushed every sibling after it onto a
new one. Keep the longest increasing run of imported keys instead, so only
the moved and inserted layers are re-keyed.
* fix(fig): don't anchor sibling keys on one nothing can precede
A moved layer in front of the lowest key ('!') left no room below it, so
the anchor was dropped and every sibling up to the next anchor re-keyed.
Such a key no longer starts the kept run when a sibling precedes it.
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