tui 2/8: read-only three-pane view & navigation

[freshtonic]

Parent

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

What to build

A read-only interactive view of the queue line: launch git queue tui, see the queue, navigate it, and read each commit's diff. No mutations yet.

• ratatui + crossterm three-pane layout (ADR-0001): queue on the left (branches as sections, commits beneath, front at top → tip at bottom); commit message top-right (read-only for now); the selected commit's own diff bottom-right, read-only.
• Diff rendering: our own +/-/hunk colouring (no syntax highlighting); binary files shown as a placeholder.
• Keyboard (vim idiom) and mouse navigation: move the selection between commits, focus a pane, wheel-scroll the diff/message/list; click a commit to select, click a pane to focus.
• A help view listing the keybindings.

Acceptance criteria

• Launching on a valid queue line renders the three panes with the queue front-to-tip and branch boundaries visible.
• Moving the selection updates the message and diff panes to the selected commit.
• The diff pane shows only the selected commit's own changes, coloured; a binary file renders a placeholder.
• Keyboard and mouse both drive selection, pane focus, and scrolling.
• A help view is reachable and lists the bindings.
• Quitting leaves the repo exactly as it was (read-only).

Blocked by

• tui 1/8: engine scaffold, git queue tui command & guards #2 (ticket 1 — engine scaffold, command & guards).