[TS] Normalize negative indices in Array.fill #437

Problem

The Array.fill approximation explicitly leaves negative start and end indices unsupported.

Minimal TypeScript example

export function fill(): number[] {
  return [1, 2, 3].fill(9, -2);
}

Expected behavior

fill() returns [1, 9, 9]. Normalize negative indices relative to length and clamp to the valid interval, including symbolic indices in the supported domain.

Current source evidence

Fill negative-index TODO.

Checked against USVM main 53dce8f3bca2713642c783ba8308a6a360a9e516. This exact snippet is a proposed regression from source audit; it has not yet been run as a new end-to-end test. #368 audits the existing call approximations but does not implement these individual fixes.

Acceptance criteria

• A focused test checks the symbolic outcome without reporting an approximation as exact semantics.
• The supported case agrees with concrete TypeScript/Node.js replay.
• Unsupported operand or environment cases remain explicit.