Treat setting a large array length as a logical length change instead of
forcing dictionary indexed storage or materializing every hole up front.
This keeps dense fills on Array(length) on the holey indexed path and
only falls back to sparse storage when later writes actually create a
large realized gap.
The asm indexed get/put fast paths assumed holey arrays always had a
materialized backing store. Guard those paths with a capacity check so
lazy holey arrays fall back safely until an index has been realized.
Add regression coverage for very large holey arrays and for densely
filling a large holey array after pre-sizing it with Array(length).
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