Squash-merged bottom PR breaks a stack; fix with an ours-merge

When the bottom PR of a stacked chain is squash-merged, GitHub retargets the next PR to trunk and marks it conflicting, because the branch still contains the original commits rather than the squash. If the squash commit's tree equals the old layer's tree (check with git diff squash old-tip), there's no need to rebase or force-push. Run git merge -s ours origin/master on the next layer (the tree stays the same), then cascade ordinary merges of each layer into the one above, asserting each tree hash is unchanged. All pushes are fast-forwards and every PR's diff stays the same. Ask reviewers to merge the rest with merge commits, not squash.