Array.prototype.{join,indexOf,map,forEach}.call(arr) does not fill holes through a custom prototype

[proggeramlug]

Summary

The generic array-like engine behind Array.prototype.<method>.call(...) reads elements on a path that bypasses array_spec_get, so a hole filled by the array's prototype is seen as a hole rather than as the inherited value.

const proto = { 1: "holeFill" };
const k = [0, , 2];              // hole at index 1
Object.setPrototypeOf(k, proto);

k[1]                              // "holeFill"  (correct after #9219)
Array.prototype.join.call(k)      // node: "0,holeFill,2"   perry: "0,,2"
Array.prototype.indexOf.call(k, "holeFill")  // node: 1   perry: -1

map and forEach diverge the same way — ES defines their hole check as HasProperty, which is a full prototype-chain walk, not an own-element test.

Not the setPrototypeOf bug

Pre-existing and independent of #9192. Found while fixing it; the control that establishes this is that the divergence reproduces identically when the prototype is itself an array — the long-supported shape. #9219 fixes direct indexed and named reads; these generic-engine element reads are a separate path it does not touch.

Impact

Silent wrong values from the most generic, most-copied array idiom in JS. Anything using Array.prototype.<m>.call on an array-like with an inherited fill — a common polyfill and arguments-handling shape — gets a hole where a value belongs.

Fix direction

Route the generic engine's element reads through array_spec_get (or the same ArrayCustomProto classification #9219 introduced in crates/perry-runtime/src/array/indexing.rs), and make the hole test a HasProperty walk rather than an own-element check when the receiver has a recorded prototype. Arrays with the default chain should keep the current fast read, gated on the existing per-array recorded-prototype check, so this costs nothing for the overwhelmingly common case.

Checks 106–110 of the differential harness in the #9219 branch already cover this.

[proggeramlug]

Closing: fixed alongside #9220 in #9370 (shared fixture test_gap_9220_9221_array_proto_paths).