refactor(examples): author controls as JSX components

[kpal81xd]

Summary

Migrates the examples' control panels from the argument-injected *.controls.mjs
factory format to developer-friendly *.controls.jsx React components.

Before — deps were passed in as arguments and JSX was hand-written via jsx()/fragment():

export const controls = ({ observer, ReactPCUI, React, jsx, fragment }) => {
    const { LabelGroup, SliderInput, BindingTwoWay } = ReactPCUI;
    return fragment(jsx(LabelGroup, { text: 'Blend' }, jsx(SliderInput, { ... })));
};

After — a named component that imports what it needs and uses real JSX:

import { BindingTwoWay, LabelGroup, SliderInput } from '@playcanvas/pcui/react';

/**
 * @param {{ observer: Observer }} props - The control panel props.
 * @returns {ReactElement} The control panel.
 */
export function Controls({ observer }) {
    return (
        <LabelGroup text='Blend'>
            <SliderInput binding={new BindingTwoWay()} link={{ observer, path: 'blend' }} />
        </LabelGroup>
    );
}

What changed

• All 89 *.controls.mjs → *.controls.jsx components (real imports + JSX).
• Controls are transpiled in the browser via a lazily-loaded @babel/standalone
  (CommonJS output + a require shim in Example.mjs). Because the raw .jsx is
  transpiled at execution time, editing controls in the code panel and reloading now
  updates the UI (previously the edit and the executed artifact diverged).
• Removed the server-side controls transpile and all legacy controls.mjs handling
  across build/dev/prod — controls.jsx is now the single controls artifact end-to-end.
• Supporting updates: eslint (.jsx + jsx-uses-vars, max-len 100), tsconfig (jsx),
  Monaco language mapping + JSDoc @scope highlighting, prettier override, README and
  templates/controls.jsx scaffold.

Integration with #8818

#8818 (World-space PCSS penumbra) landed concurrently and modified controls this branch
migrated. Its changes are re-applied in the new format so nothing regresses:

• graphics/shadow-soft: Penumbra slider 1/100/0 → 0/0.2/3
• graphics/shadow-cascades: added PCSS Penumbra + PCSS Falloff sliders
• gaussian-splatting/shadow-soft (new example in [BREAKING] World-space PCSS penumbra, plus gsplat & cascade fixes #8818): its controls migrated to JSX

Verification

• npm run lint and npm run type:check:examples — pass.
• Browser-pipeline harness: all 90 controls.jsx transpile (automatic runtime +
  commonjs) and load to a Controls export.
• Runtime (dev server): controls render across examples; live-edit verified (edit a label
  → reload → UI updates); graphics/shadow-cascades shows the new PCSS sliders.
• Adversarial parity review of the one non-mechanical rewrite
  (blend-trees-2d-cartesian, class → useRef/useEffect): no behavioral regressions.

Note: @babel/standalone (a pre-existing, previously-unused devDep) is now a
lazily-loaded chunk fetched the first time controls build — the necessary trade-off for
in-browser JSX editing.

[mvaligursky]

Nice!