Ctrl+\ puts the current file beside the others, in a second editor group #460

Context

The editor area holds two editor groups side by side, each with its own tabs, so a test can sit beside the code it covers. A tab moves between groups; a file is open in at most one. Commands act on the focused group. Pitch: #60.

This waits for #451, which reworks how files open; this task changes the same open path to respect "one file, one tab".

Acceptance criteria

• With two files open, Ctrl+\ (or mg, or View → Move to other group) moves the active tab into a new group on the right. The editor area splits its width evenly, each group with its own tab strip.
• Ctrl+\ on a tab in the right group moves it back to the left. Moving a group's only tab out, or closing a group's last tab, closes that group and the other takes the full width.
• mg on the only open tab, with no split, says Open another file to split with in the status bar and changes nothing.
• F6 (or fo, or View → Focus other group) moves focus to the other group's active tab. The focused group is drawn with the focused border. Clicking in a group focuses it.
• jg (or View → Join groups) moves every tab back into one group.
• Opening a file that's already open in the other group, from the Explorer, Ctrl+O, a Find result, Back/Forward, or the Review tab, focuses it there instead of opening a second copy. A new file opens in the focused group.
• In the focused group, these act on its active tab: save, close, the find bar, go to line, Ctrl+1…9, next/previous tab, change grammar, document info (di), blame (Ctrl+G B) and the Compare commands. The status bar's path, Ln, Col and selection count follow it, and so do multi-cursor carets.
• Save all saves the dirty tabs in both groups, and quitting with unsaved changes in either group asks first.
• Renaming, moving or deleting a file in the Explorer updates or closes its tab in whichever group holds it. A file changed on disk shows the usual notice in either group.
• Find in files with open buffers, and Replace all, cover the tabs in both groups.
• Back/Forward (cursor history) jumps into the group that holds the file.
• A diff tab or the PR overview can be moved to either group like any tab.
• Quit with a split and reopen the folder: both groups come back with their tabs, and the same group is focused. A folder last opened before this update opens as one group.
• F1 and the command palette list Move to other group, Focus other group and Join groups with their keys.

Tests

• Moving tabs between groups: split, move back, unsplit on emptying a group, mg with one tab and no split.
• Focus: which group the active-tab commands reach after fo, a click, and closing the focused group.
• One file, one tab: opening an already-open file from each entry point focuses the existing tab.
• Save all, quit-with-unsaved, rename/move/delete, disk-change notices and global find/replace across both groups.
• Workspace state round-trip with a split, and loading a state file from before this change.
• Mnemonics mg, fo and jg are free and no prefix of another.

Out of scope

• The same file in both groups, or more than two groups.
• Stacked (top/bottom) groups, or stacking automatically at narrow widths.
• Dragging tabs between groups, or resizing the split with the mouse.
• Anything diff-specific.