Match tsc for JSX uppercase hex entities

[Copilot]

tsgo decoded JSX numeric hex entities with an uppercase marker (&#X...;), while tsc leaves them literal. This caused JSX text and attribute emit to differ from TypeScript.

• JSX entity decoding

  • Treat only lowercase x as the hexadecimal numeric entity marker.
  • Leave &#X...; unchanged by falling through as a non-entity.

• Regression coverage

  • Added a compiler test covering uppercase hex markers in JSX text and attributes.
  • Kept coverage for lowercase hex and decimal entities to ensure they still decode.

export const a = <div title="&#X42;">&#X41;</div>;
// emits: React.createElement("div", { title: "&#X42;" }, "&#X41;")