runtime: honor flat depth in dynamic Array.prototype.flat dispatch #2800

Summary

Perry has a depth-aware Array.prototype.flat(depth) lowering path, but the dynamic runtime method-dispatch branch ignores the supplied depth argument and always calls the legacy depth-1 helper. Calls that reach dynamic dispatch, such as anyArray.flat(0), anyArray.flat(2), or anyArray.flat(Infinity), should preserve Node's depth coercion semantics.

Node parity probe

Run with Node v25.9.0:

function nest(depth) {
  let x = "leaf";
  for (let i = 0; i < depth; i++) x = [x];
  return x;
}
for (const depth of [0, 1, 2, 32, 33, 40, Infinity, NaN, -1]) {
  const out = nest(40).flat(depth);
  let d = 0, cur = out;
  while (Array.isArray(cur)) {
    d++;
    cur = cur[0];
    if (d > 50) break;
  }
  console.log(`${String(depth)}: residualArrayDepth=${d} leaf=${cur}`);
}

Output:

0: residualArrayDepth=40 leaf=leaf
1: residualArrayDepth=39 leaf=leaf
2: residualArrayDepth=38 leaf=leaf
32: residualArrayDepth=8 leaf=leaf
33: residualArrayDepth=7 leaf=leaf
40: residualArrayDepth=1 leaf=leaf
Infinity: residualArrayDepth=1 leaf=leaf
NaN: residualArrayDepth=40 leaf=leaf
-1: residualArrayDepth=40 leaf=leaf

The top-level result array accounts for the final residual depth of 1 in the fully flattened cases.

Current Perry behavior in source

The static/codegen path is depth-aware:

• crates/perry-codegen/src/lower_array_method.rs:198-218 sends zero-arg flat() to js_array_flat, but sends calls with an argument to js_array_flat_depth(recv, depth).
• crates/perry-runtime/src/array/flat_clone.rs:31-47 converts NaN / non-positive depth to 0, finite positive depth to an integer depth, and Infinity to u32::MAX.

But dynamic method dispatch ignores the argument:

• crates/perry-runtime/src/object/native_call_method.rs:1243-1246 handles "flat" by calling crate::array::js_array_flat(arr) unconditionally and never reads args_ptr / args_len.

So any call that falls through to dynamic method dispatch gets depth 1 regardless of whether the JS call supplied 0, 2, Infinity, or another value.

Expected fix shape

The dynamic flat branch should mirror the static path:

• Omitted depth: call js_array_flat or js_array_flat_depth(arr, 1).
• Supplied depth: pass the first argument to js_array_flat_depth after normal JS number coercion.
• Preserve Node behavior for 0, negative values, NaN, finite positive depths, and Infinity.

Please add regression coverage for dynamically dispatched flat(0), flat(2), flat(Infinity), and the zero-argument default.