Make graph editing stepped and spatially stable

[Today20092]

What to build

Replace auto-fitting graph movement with a stable exposure workspace and one visible increment choice shared by direct manipulation, scrolling, and keyboard editing.

Acceptance criteria

• Stops mode keeps a fixed -7 to +7 viewport while bands are edited.
• IRE mode keeps a fixed 0 to 100 viewport.
• A visible increment control offers 1, ½, and ¼ stop choices in Stops mode.
• Dragging, wheel movement, and arrow-key movement use the selected increment consistently.
• Ctrl or Meta wheel remains available to the browser and does not edit a band.
• Values outside the visible stop range remain editable and receive a clear edge/out-of-view indication instead of resizing the axis.
• Width editing remains independently usable and does not cause the viewport to resize.

Blocked by

None — can start immediately.