stacked-merge ledger — PRs merged down-stack, not yet on main

[github-actions]

PRs merged into a non-main base. Each row is a stranded head SHA; the push-to-main job checks each off once it becomes an ancestor of main. Auto-closes when all rows land.

• feat(borrowck): HX3055 L5-E15 assign-to-field/index-of-immutable-owned-base (E0594) #4822 · base fleet/memory-hx3052 · head 66f9252374dc94e25da771f0d0c5b6a461d87bf3

• feat(check): HX3054 arr[aggregate] index-type REJECT (E0277) — static-types R4 #4823 · base fix/static-hx3053-arr-str-index · head 31ae119a755982b5d6dae1581401d81b2a4dfbe2