The main-checkout test compared the two probe roots with raw string
equality. When they differed only in separator spelling, the repo's own
checkout was misclassified as a linked worktree: it fell through to the
worktree branch and was labeled by directory basename. The sidebar then
showed one checkout twice — a dir-labeled lane plus the branch-labeled
`main` lane built from the same sessions.
Compare with `_path_key` so platform path identity decides, matching how
every other path comparison in this module is already keyed.
Tests cover the single-checkout case and the main + linked-worktree case;
both fail before this change (the lane comes back labeled `repo`, not
`main`).
