Add permanent text labels to Sankey links

What

Adds persistent, always-visible text labels to Sankey links, analogous to the existing node
labels. Five new attributes under link:

Why

Link information is currently hover-only, making Sankey diagrams unsuitable for static export
(PNG/SVG/print) wherever flow values or names need to be visible at all times. There is
long-standing community interest in this — see #4746.

External SVG overlays (the current workaround) are fragile: they break on node drags, require
timing hacks after layout transitions, and are excluded from toImage export. Native labels
solve all three problems.

How

Labels are rendered as real SVG <text> elements inside the .sankey group, immediately after
the link paths. This means:

• The existing group-level orientation transform applies automatically — labels stay upright in
  h and v layouts with no extra handling.
• Labels follow node drags via the existing updateShapes hook — no separate drag listener
  needed.
• Labels are included in toImage/PNG/SVG export — no overlay required.
  For circular (backward) links the label anchor reuses the same circularPathData midpoint as
  the hover label, so the position is consistent. Vertical centering on the anchor uses
  dominant-baseline="central".

Template rendering follows the established funnel/pie pattern via Lib.texttemplateString.
Available template variables: %{label}, %{value}, %{valueLabel} (formatted value +
suffix), %{source}, %{target}, %{customdata}.

Backward compatibility

Fully backward compatible. All new attributes default to textinfo: 'none' / texttemplate: '',
so existing figures render identically. No existing tests or baselines are affected.

Related

• Closes / addresses add optional text on Sankey links #4746 (add optional text on Sankey links)