Function withIconOverride

withIconOverride(
    scale: IconSet,
    band: number,
    icon: { id: number; set: IconSet } | null,
): IconSet
Returns the rule with one band's icon replaced.
Parameters
- scale: IconSet
  The rule to copy from. Not mutated.
- band: number
  The band to change, 0 being the lowest.
- icon: { id: number; set: IconSet } | null
  The icon to draw there, or null to restore the band's own.
Returns IconSet

Remarks
The override array is kept full or absent, never sparse. Excel writes an x14:cfIcon for every band once any is custom (custom="1"), so a partial array has no wire form; and a rule whose every band still draws its own icon carries no array at all, which keeps it a legacy (non-x14) rule until the user actually changes something.