BlockSection::get and set check the section index and the state id once,
then read and write through new PackedStorage::read/write, which skip the
storage-level bounds and value checks and the Result that get/set keep for
external callers. A section index is below 4,096 and a palette id always
fits the width bits_for_palette chose; both methods debug_assert that
contract. Used for the hot read, the hot write and the growth re-encode.
Benchmark (ignored tests, best of alternating-order rounds, fast profile):
BlockSection::set 13.15 -> 9.12 ns/write (1.44x), get 2.33 -> 1.45 ns/read
(1.61x). See docs/performance/paletted-container-access-results.md.
rdbtCVS
5c55e7d855