tui 8/8: split — line-level interactive selector, two-way repeated

[freshtonic]

Parent

#1 — git queue tui — interactive queue/history editor (TUI)

What to build

Split one commit into several via an interactive, line-level selector. (Runs parallel to delete/squash; needs the conflict machinery.)

• The diff pane gains an interactive line-selection mode for the selected commit: toggle individual added/removed lines (keyboard and mouse) into "this piece" vs "the rest".
• Two-way primitive, repeated: each split produces two commits; repeat on the remainder to reach N pieces.
• Identity (follow jj): the older resulting piece keeps the original Stable-Commit-Id and message; each peeled piece gets a freshly minted id and a message the user writes. Any commit the TUI creates is auto-stamped so the queue never leaves with untracked commits.

Acceptance criteria

• The diff pane enters a line-level selection mode; lines can be toggled by keyboard and mouse.
• Closing the selection splits the commit into two, with the selected content in one and the rest in the other.
• Repeating on the remainder yields three or more pieces.
• The older piece keeps the original id + message; peeled pieces get fresh ids and written messages; all new commits are stamped.
• Split is undoable.
• Engine integration tests cover a two-way split (content division + id/message assignment) and a repeated split producing three pieces.

Blocked by

• tui 4/8: reorder + conflict escape hatch (resolve/undo, suspend/resume) #5 (ticket 4 — reorder & conflict machinery).