History window: the whole timeline, Undo and Redo

[AliceLafox]

The slot tab (#50) answers "what happened to this slot". This one answers "what
happened at all": one timeline across every slot and every connection, sharing
the tab's model and rows so the two can never drift into two different truths.

• Rows carry the time, the action, the slots it touched, the words for what
  changed, and the state of the take behind it: kept, audio no longer kept,
  recorded on the pedal, interrupted.
• Filter by slot (clicking a slot badge filters to it), pin a row so freeing space
  (History storage: show what it costs, free it on request #74) never releases its audio, and Export take to... — the bytes live in the
  store now, so the app writes a file where the user asks instead of revealing a folder.
• Connection markers sit in the timeline like any other row: "plugged in — the
  pedal brought slots 12 to 14".

Undo and Redo

In the Edit menu, on Cmd-Z and Cmd-Shift-Z, with the target named on the item so
the keystroke is never a guess.

An undo is a new row, never a stack pop. The bytes really were written to the
pedal, and a history that pretended otherwise would be lying about a write that
happened. Redo is the undo of that undo, and it is a row too. Nothing is ever
rewritten or cut from the timeline: picking a row in the middle and restoring
from it writes forward, on top, like everything else.

Crossing a connection marker, or a row that records a change made on the pedal
itself, is allowed — the history is one continuous line, and "put back what was
there before I unplugged" is a real request. But the first press that crosses one
shows what it is about to discard. A speed bump, not a wall.

Undo is ours as a word for the app's own timeline; the pedal's UNDO/REDO stays
the pedal's, and the guide keeps them apart.

Disabled while the pedal is busy or away, like every other mutation.