The Extensible Array bug reached a release because no fixture had more
chunks than fit inline, so its data blocks were never read. The Fixed
Array index had the same blind spot: nothing exercised it above a
handful of chunks, and nothing reached the paged layout at all.
Checked at 100, 5 000 and 200 000 chunks plus a sparse dataset that
leaves whole pages uninitialised. It is correct throughout — it does
keep its page-init bitmap inside the data block, which is the
difference from the Extensible Array that made assuming otherwise a
bug. Adding the tests so that stays true.
Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
