refactor: store bitpacked chunk widths in a child

[mhk197]

This change is now reviewed in #10004.

During the branch reorder, GitHub automatically marked this PR merged into the kernel branch. That state change did not merge these changes into develop.

[codspeed]

Performance comparison unavailable

The base and head of this comparison were measured with different runner settings, so their benchmark values are not directly comparable.

What changed between base and head:

• Allocator-time exclusion (off → on) removes allocator time from the measurement, which shifts benchmark values. Learn more about allocator-time exclusion

Re-run the base with the same settings to get a valid performance comparison.

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_{Comparing mk/bitpacked-stack-03-width-child (afae8b0) with mk/bitpacked-stack-02-cpu-layout (844cc78)¹}

Footnotes

1. No successful run was found on mk/bitpacked-stack-02-cpu-layout (bd67461) during the generation of this report, so f3f450b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩