[TS] Preserve lexical this of arrow function values in EtsIR #391

Problem

The native frontend lifts an arrow function and an ordinary function expression through the same closure path. The lifted EtsIR method does not retain that the arrow's this is captured from its creation site. When the function value is stored on another object and called there, this distinction is required for correct execution.

Minimal TypeScript example

class Box {
  callback!: (value: number) => number;

  constructor(public offset: number) {}

  installOn(target: Box): void {
    target.callback = (value: number) => this.offset + value;
  }

  call(value: number): number {
    return this.callback(value);
  }
}

export function lexicalThis(): number {
  const source = new Box(11);
  const target = new Box(100);
  source.installOn(target);
  return target.call(1);
}

lexicalThis() must return 12, because the arrow captures source as this; an ordinary function expression called through target.callback would use the call receiver instead.

Source evidence and observed consumer failure

• Closure lowering uses the same lifted-method representation for arrows and function expressions. It changes isStaticMethod for arrows, but emits no function-kind marker or captured lexical receiver.
• Method prologue loads this from ThisRef in the lifted method.
• [TS] Support object arrays in symbolic input fields usvm#419 originally recorded the related stored-arrow test on the [TS] Lower array rest bindings with iterators (#383) #418 head: expected 12, observed 101. That test was moved out of [TS] Execute parameter defaults in declaration order (#372) #419 so its object-array input gap remains one issue.

The source links are pinned to JacoDB neo 873f8e99. The standalone snippet compiles with TypeScript and returns 12 under Node.js. It has not been run as a new JacoDB frontend regression; the reported 101 came from the related #418 test.

Acceptance criteria

• EtsIR distinguishes arrows from ordinary function expressions and preserves the lexical receiver of an arrow through JSON round trip.
• A focused frontend regression checks the distinction in the stored-function case above.
• A consumer can call the stored arrow and obtain 12 while ordinary functions continue to use their call receiver.