runtime: implement Array.prototype.toLocaleString element dispatch #2808

Summary

Array.prototype.toLocaleString(locales?, options?) should call each non-nullish element's own toLocaleString(locales, options), join the results with commas, and render null / undefined elements as empty fields. Perry installs the Array prototype method name, but source search does not show an Array-specific implementation or dynamic dispatch branch.

Node parity probe

Run with Node v25.9.0:

const obj = {
  toLocaleString(locales, options) {
    return `obj:${locales}:${options && options.tag}`;
  },
};
for (const [label, fn] of [
  ["plain", () => [1, null, undefined, "x"].toLocaleString()],
  ["locales/options forwarded", () => [obj].toLocaleString("xx-YY", { tag: "opt" })],
  ["number locale", () => [1000.5].toLocaleString("en-US")],
  ["date locale", () => [new Date(Date.UTC(2020, 0, 2))].toLocaleString("en-US", { timeZone: "UTC" })],
]) {
  try {
    console.log(label + ": " + JSON.stringify(fn()));
  } catch (e) {
    console.log(label + ": throws " + e.name + ": " + e.message);
  }
}

Output:

plain: "1,,,x"
locales/options forwarded: "obj:xx-YY:opt"
number locale: "1,000.5"
date locale: "1/2/2020, 12:00:00 AM"

Current Perry behavior in source

The method is exposed but there is no visible Array implementation:

• crates/perry-runtime/src/object/global_this.rs:951 installs ("toLocaleString", 0) on Array.prototype.
• Source search for ArrayToLocale, ArrayLocale, and toLocaleString finds no Array HIR node, no js_array_to_locale_string helper, and no Array dynamic branch in crates/perry-runtime/src/object/native_call_method.rs.
• The only generic runtime toLocaleString fallback I found is for primitives: crates/perry-runtime/src/object/native_call_method.rs:1879-1889 delegates primitive value.toLocaleString() to toString().
• Date/number-specific toLocaleString lowerings exist elsewhere, but they do not implement the Array algorithm that calls each element's own method with (locales, options).

So array.toLocaleString(...) currently has no Array-specific path to walk elements, skip nullish values, and forward locale/options arguments.

Expected fix shape

Implement Array-specific toLocaleString semantics:

• Iterate the array from 0 to length - 1.
• For null / undefined / holes, append an empty string.
• For other values, call that element's toLocaleString(locales, options) and stringify the result.
• Join element strings with comma separators.
• Preserve locale/options forwarding for objects, numbers, dates, and other values.

Please add regression coverage for nullish elements, custom element toLocaleString, number/date elements with locales/options, and dynamic dispatch through an any receiver.