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ALTREP for owned const-generic arrays.
The shared family generators keep native pointers and materialized data2 behaviour consistent with Vec/Box. Each element family caches a separate class per N: a static inside a generic function alone would be shared by every N and install callbacks for whichever length registered first.
Classes register on first conversion. Duplication and serialization keep R’s default ordinary-vector fallback, so restoring a value never depends on having instantiated its array length in the receiving R session.