Fix lost writes after ?? and || defaults

[KayleeWilliams]

I hit this moving our CLI (inthhq/inth) to 0.2. parseArguments passes result to a function that sets skillsArguments, and on 0.2 the caller never sees the write. Smaller repro:

interface Args { command: string; extra?: string[] }
const fill = (options: Args, values: string[]): void => { options.extra = values; };
const parse = (args: string[]): Args => {
  const result: Args = { command: "" };
  for (let i = 0; i < args.length; i += 1) {
    if (args[i] === "skills") { fill(result, args.slice(i + 1)); return result; }
  }
  result.command = args[0] ?? "";
  return result;
};
console.log(parse(["skills", "--yes"]).extra);
// Node: [ '--yes' ]
// 0.2.1: undefined

mayBeOptional treats a ?? b and a || b as optional if either side is, but both give b when a is undefined. So args[0] ?? "" counted as optional, result got a widened shape, and the call to fill went through a %rec.width copy. This checks only the right side for ?? and ||.

nullish-default-record-writes.ts fails on main and passes with this. The rest of the corpus passes too, except 1640-fd-read-decode.ts and 2283-structured-clone.cjs, which fail on main for me as well (Node 24.19). I didn't run the sanitized lane.

The copy itself is still there. With result.command = args[0] and no default, the write is still lost. I wasn't sure how you'd want that fixed, so I left it alone.

[vercel]

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