runtime: add dynamic Array.prototype.copyWithin dispatch

[andrewtdiz]

Summary

Array.prototype.copyWithin(target, start?, end?) mutates and returns the receiver for all valid argument counts with a supplied target. Perry has a static/codegen path for this method, but dynamic runtime method dispatch still has no copyWithin branch, so calls that fall through to dynamic dispatch do not reach js_array_copy_within.

Node parity probe

Run with Node v25.9.0:

for (const [label, fn] of [
  ["copyWithin target only", () => [1, 2, 3, 4].copyWithin(1)],
  ["copyWithin target start", () => [1, 2, 3, 4].copyWithin(1, 2)],
  ["copyWithin target start end", () => [1, 2, 3, 4].copyWithin(1, 0, 2)],
  ["copyWithin neg", () => [1, 2, 3, 4].copyWithin(-2, 0, -1)],
]) {
  try {
    console.log(label + ": " + JSON.stringify(fn()));
  } catch (e) {
    console.log(label + ": throws " + e.name + ": " + e.message);
  }
}

Output:

copyWithin target only: [1,1,2,3]
copyWithin target start: [1,3,4,4]
copyWithin target start end: [1,1,2,4]
copyWithin neg: [1,2,1,2]

Current Perry behavior in source

The static/codegen path exists:

• crates/perry-codegen/src/lower_array_method.rs:160-195 handles copyWithin, including the one-argument form by defaulting start to 0 and the omitted end form by passing has_end = 0.
• crates/perry-runtime/src/array/immutable.rs:198-257 implements js_array_copy_within(target, start, has_end, end) with negative-index normalization, end clamping, and memmove semantics.
• crates/perry-hir/src/lower/expr_call/local_array_methods.rs:508-520 has an HIR fast path for local array receivers with at least two arguments.

But dynamic runtime method dispatch does not include a copyWithin branch:

• crates/perry-runtime/src/object/native_call_method.rs has dynamic branches for nearby mutating array methods such as sort, reverse, reduce, flat, flatMap, fill, and others, but source search finds no "copyWithin" branch.
• The static codegen comment at crates/perry-codegen/src/lower_array_method.rs:161-167 explicitly notes that the general method-dispatch fallback does not know about copyWithin and silently no-ops.

So any copyWithin call that reaches dynamic dispatch, including calls on any/unproven receivers, misses the runtime helper even though the helper already exists.

Expected fix shape

Add a dynamic dispatch branch that mirrors the static path:

• Require at least the target argument.
• Default omitted start to 0.
• Preserve omitted end with has_end = 0; otherwise pass the supplied end.
• Call js_array_copy_within and return the original receiver pointer.

Please add regression coverage for dynamically dispatched copyWithin(target), copyWithin(target, start), copyWithin(target, start, end), negative indexes, and overlapping copy regions.