Static image rendering for Swing and JavaFX components

[mkhawam]

Summary

The portable, JupyterLab-independent first slice of #122: render GUI components as static images via
display(...), for both Swing/AWT and JavaFX. No frontend, no comms, no extension — just images
(viewable anywhere a PNG is: GitHub, nbviewer, etc.).

What's included

• java.awt.Component → PNG. Painted off-screen into a BufferedImage and encoded by the existing
  Image pipeline. Headless-native (no native window is created).
• javafx.scene.Node → PNG. Snapshotted on the JavaFX Application Thread
  (Node.snapshot(...) → SwingFXUtils.fromFXImage → BufferedImage). Done reflectively, so
  jjava-jupyter keeps no compile- or runtime dependency on JavaFX — it's optional and user-supplied
  (e.g. %maven org.openjfx:javafx-controls:...). If the toolkit can't start, it degrades to a short text
  message instead of failing the cell.
• Name-based renderer registration (Renderer.createRegistration(String) / registerByName). A
  user-supplied JavaFX node is loaded by the JShell class loader, not the kernel's, so Class-identity
  registration can't match it; matching by fully-qualified class name (against the value's class hierarchy)
  is class-loader agnostic. This is a small, generic addition useful for any optional runtime type.

So display(component) / display(node) (and a bare last-expression value) now produce a still image.

Headless note

JavaFX snapshot() needs the JavaFX toolkit, which needs a display. On a headless server/CI, run the kernel
under Xvfb (xvfb-run). (Monocle is not a viable route on JavaFX 21 — OpenJFX 21 doesn't ship
MonoclePlatformFactory.) The Swing renderer needs none of this.

Tests

Headless unit tests: SwingTest (Component → PNG of expected size), RendererTest (name-based dispatch),
JavaFxTest (graceful fallback when JavaFX is absent). Full jjava-jupyter module green.

Relation to #122

This is the portable core split out of #122 per the suggestion to digest it one piece at a time. The
interactive parts (comm-streamed frames + mouse/keyboard, and the JupyterLab extension) stay in #122 as a
draft for a later, separate PR.

[mkhawam]

Another question is JavaFX requires JVM Options to run in headless mode. It a bit of a pain to add the -Dglass.platform=Monocle -Dmonocle.platform=Headless at the end of the kernal.json. It would also be a pain to have another installation process just for a user that wants to use JavaFX.

Although JavaFx does give smoother interaction then swing.

[andrus]

JJAVA_JVM_OPTS to the rescue. I never really modify kernel.json in my own install.

[mkhawam]

That means it will not work out of the box. Might want to it to the docs or outputting a warning if someone wants to use javafx if those options are not enabled?

[andrus]

Disclaimer: I need more time to get a better feel of how the whole things works to comment intelligently.

But maybe we make the whole thing an optional kernel extension that is loaded explicitly (by adding it as a dependency), and this is where it can complain about missing options.