fix(sankey): transpose node.x/node.y scaling for vertical orientation (Fixes #7947) - #7957

Problem

For vertical Sankey traces (orientation: 'v'), user-supplied node.x / node.y are normalized against the wrong extent. The d3-sankey layout space always carries the flow along its x axis, but for vertical orientation that space is transposed onto the screen. When the plotting area is non-square, nodes are placed outside the bounding box and the values written back after a drag can exceed the documented [0, 1] range.

Root cause

In src/traces/sankey/render.js, sankeyModel():

sankey.size(horizontal ? [width, height] : [height, width]);   // (1) correctly transposed
...
var pos = [trace.node.x[i] * width, trace.node.y[i] * height]; // (2) NOT transposed

The user coordinates at (2) are scaled by the un-transposed extents, while the d3-sankey layout at (1) uses the transposed extents. The same asymmetry exists in the write-back path persistFinalNodePositions().

Fix

1. In sankeyModel(): scale node.x by the flow-axis extent and node.y by the cross-axis extent, matching the sankey.size() transposition
2. In persistFinalNodePositions(): divide by the same transposed extents on write-back

Closes #7947