Both factories know what they built, but returned the bare proxy, so a
caller reading componentProperties, setProperties or isExposedInstance
had to narrow first — the instance surface is only spelled out on the
node types. Two test suites had each grown their own cast for it.
FigmaInstanceNode joins the other node types and is exported, and the
compatibility check names it instead of respelling the intersection.
Narrowing a return type is not a breaking change: a caller that held
the result as a FigmaNodeProxy still compiles.
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