implement MultitabPane widget + controller (T-83)

MultitabController<T> is a Flutter-free ChangeNotifier owning the
tab list, active selection, and reorder/close invariants:
- pinned (non-reorderable) entries form barriers that other tabs
  cannot cross
- non-closeable entries silently no-op on remove() so hosts don't
  need to gate the call site
- closing the active tab falls right, then left, then to null
- duplicate ids are rejected

MultitabPane<T> is the widget shell: a horizontal tab strip
followed by the active entry's body. Active tab gets the
panelHeader background and a panelActiveBorder top accent;
inactive tabs blend into the tab bar. Close × is hidden until
hover. Add button only renders when onAddRequested is wired.

Hosts route the user's add/close intent through callbacks so the
widget stays domain-free — for the Claude pane, add will spawn a
new tmux session and close will kill one. Drag-to-reorder is
controller-side only for now (the gesture wiring lands with T-24).

19 controller tests + 9 widget tests.

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -23,6 +23,11 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   history naturally.
 - Welcome screen Tips card — six common keybindings shown below the
   START / RECENT row when the viewport is tall enough.
+- `MultitabPane` widget + `MultitabController` for panes that host
+  N runtime tab instances of the same kind. Generic over a payload
+  type, supports pinned/non-closeable tabs (primary), drag-reorder
+  (planned), close × on hover, and an optional `+` add button. Used
+  by the Claude pane to render primary + secondaries.
 
 ### Changed