Per-Compound Iteration Stepper
Each loop/split compound gets its own stepper in the header bar
Nested Loops
Each compound level has its own independent stepper
Stepper Behavior
- Independent per compound — outer loop at iteration 2, inner split at branch 1
- Overlay updates per-compound — stepping the loop re-renders its children's execution data for that iteration
- CHOICE shows which branch was taken — no stepper, just highlights the taken branch
- Keyboard — when a compound is focused/hovered, left/right arrow keys step through iterations
- Detail panel syncs — selecting a processor inside a loop shows that iteration's data