Function findNextCellWithinSelection

Cycles the active cell to the next cell within the current selection, the Tab/Enter traversal (it never changes which ranges are selected — only the active cell and active range index). Semantics:

• wraps at a range edge to the next row/column of that range;
• visits each cell once (steps over a merged cell after its first visit);
• jumps to the next range (cycling) when stepping off the last corner of the active range;
• skips hidden rows/columns via the optional ports.

Pure: it holds no sheet/grid state — merged cells and hidden headers come in through the GetRangeFromCell / HiddenHeadersAt ports. The merge-unaware caller passes CoordUtils.cellToRange and no hidden ports. Returns the new SelectionCoords (the ranges reference is preserved), or selection unchanged when the active cell is outside the active range.