layout: placement-followups - #878

work#312 placement follow-ups (1)-(3): a sash drag while turned writes nothing, besideGrouping sends only to a shown pane, Workspaces.sync builds one Layout per pass; +114/-19 with tests for (1) and (2). My 13:35Z past-round-two decision stands; M1 (merge main) verified clean at 7ad6abf by macos/agy (author macos/claude; one conflict in tests/integration.zig, both tests kept). Full CI green at head. Item (4), the place commands, was optional and is not wanted now. Closes work#312.