Add bottom-up and right-left orientations for Sankey diagrams

[wf-r]

What

See #7865

Adds two new flow directions to Sankey traces and gives the existing horizontal/vertical modes explicit, direction-named aliases, fully backward compatible.

orientation now accepts:

• h — horizontal, default (unchanged)
• left-right — sources left, flow rightward (synonym of h)
• right-left — sources right, flow leftward (new)
• v — vertical (unchanged)
• top-down — sources top, flow downward (synonym of v)
• bottom-up — sources bottom, flow upward (new)

Why

So far, h is always left→right and v is always top-down. Right-to-left and build-up (bottom-up) flows are common conventions with no equivalent today. This closes both gaps additively, without changing any existing rendering.

Backward compatibility

Purely additive. Existing figures using h/v render identically; the default is unchanged.

[camdecoster]

I'm going to change the target branch from master to our upcoming major version branch, v4.0. We've some changes in the Sankey code and I'd like to add your changes against that. If you have time, it would be great to get this in for that version. I'll leave you a review tomorrow.

[wf-r]

Thanks for having a look (I have rebased on v4.0 for the moment)