Verifying each branch of a stack: use a detached scratch worktree

To build and test every branch of a stacked-PR series independently, add one extra worktree (git worktree add --detach <dir> <base>) and loop git checkout -q --detach <branch> && git clean -fdxq <subdir> in it. Checking out the branch by name (without --detach) makes git lock that branch to the scratch worktree, so a later checkout or rebase of it from your main worktree fails with 'already checked out'. git clean -fdx on the assignment subdir between branches keeps stale build products from one branch masking a broken build in the next. Run the loop in the background with one log line per branch, since a long foreground run can get killed and lose all results.