Spike: how to keep the node icon legible at high background opacity

## Goal

Decide how a node icon should stay legible as its background opacity approaches 100%, where a solid background color currently obscures the icon (#1994). This is a prototyping spike: produce a recommendation and a rough prototype to react to, not a shipped fix.

Questions to resolve:

- **Threshold approach** — does switching the icon to white or black once background opacity passes a threshold read well across the range of background colors and icons? Where does the threshold sit, and how is the white-vs-black choice made (e.g. background luminance)?
- **Compositing approach** — can the icon be drawn above the background layer so opacity applies to the fill only, leaving the icon untouched? What does this cost in the Cytoscape node style mapping (`toCyNodeStyle`)?
- **Separate control** — is a distinct icon-opacity (or icon-color) control warranted instead of deriving behavior from background opacity?
- **Interaction with the styling model** — how does the chosen approach fit the direction in #1943 (Improved Style Options), so we do not build something the style revamp will immediately rework?

## Related Issues

- Parent #1994 — the bug this spike scopes a fix for
- Related to #1943 — Improved Style Options roadmap

---

> [!IMPORTANT]
> Internal only — this issue is maintained by the core team and is not accepting external contributions.

Start by examining the Cytoscape node style mapping entry point, toCyNodeStyle, and the direction in #1943. Prototype the threshold, compositing, and separate-control options against varied background colors and icons; done means a recommendation and rough prototype that address the listed questions without shipping a fix.

Written by the indexing model from the issue text.