A dirty tree is now three-way merged (current check-in, working file, target) rather than rejected, so a branch tip that advanced under uncommitted work can be joined. Previously update, merge and stash all refused that state, leaving commit --allow-fork as the only exit -- the guard that keeps forks deliberate was manufacturing them. The fork guard itself is unchanged.

applyCheckinWith scans the tree once and calls back for the merge policy; handing carries in instead would scan every clean update twice, which cost a 20k-file no-op update 63%.

Paths that cannot merge either way (type change, edited here / deleted there) are named and refused before anything mutates.

Fixes #135 closed check-in: 7fcc9b5ee1 user: dmestas tags: update-dirty-tree-135