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perf(stack): read a stack's members back together, not one at a time master

`resubmit` and `merge` each read every member with one `.await` per loop iteration, and every read is a `getRecord` plus a patch blob download, so a ten-member stack was ten round trips before either could plan. Both go through `read_members`, `buffered` and so still in chain order. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>


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