Arrays longer than 2^20 kept every element beyond the cutoff in a sorted
side store, and each indexed write to an existing entry deleted and
reinserted it, so fill + indexed read/write loops went quadratic
(n=1.2M: 10.4s on macOS arm64, 12.6s on Linux x64).
- Rewrite existing sparse entries in place.
- Let dense storage grow past the cutoff for contiguous extension of a
populated prefix, bulk range writes, and sparse tails that reach 3/16
density, migrating side-store entries so index < cap stays dense.
- Make the side store a deque with front slack so descending first-touch
writes and prefix migration are amortized O(1).
- Bound range-derived slice/splice capacity hints for hole runs.
scr_arr_new now reserves the live count, so a 2^20+3 element Set
converts into dense storage instead of a sparse tail; the white-box
test_lib check that pinned the old policy now asserts dense storage.
Malte Ubl
e9f81cd064