[TS] Execute export-default expressions during module initialization #378

Problem

ExportAssignment is skipped while lowering the module body, so an exported expression is not evaluated.

Minimal TypeScript example

let count = 0;
function sideEffect(): number { count++; return count; }
export default sideEffect();

Expected behavior

Module initialization calls sideEffect() once and exports the returned value 1. Preserve its side effects and the exported binding in EtsIR.

Current source evidence

Export assignment handling; export metadata.

Checked against current JacoDB neo 873f8e99faf3f1e4736935be96ce1c319985c574. This exact snippet is a proposed regression from the source audit; it has not yet been run as a new end-to-end test.

Acceptance criteria

• The native TypeScript frontend emits EtsIR preserving the behavior above, including a JSON round trip.
• A focused regression checks the observable result against TypeScript/Node.js.
• Until implemented, the construct remains an explicit unsupported outcome; it is not silently represented as ordinary supported behavior.