[wieslawsoltes]

Problem and change

Code OSS calls children.item(index) and childNodes.item(index), while WebScene returned arrays without the DOM collection method.

This change installs the existing bounded collection item callback and covers element filtering, node ordering, wrapper identity, and out-of-range null. It is one focused commit above #125.

Closes #80. The equivalent commit remains in draft consolidation PR #76.

Cumulative validation

• Base layer b32eaf64; exact cumulative head 891e63bb
• Pinned WebIDL generator consistency: 21 interfaces and 180 members; generated catalogs byte-identical
• Release V8 native engine build
• Cumulative dom-traversal-cloning-primitives contract: 10/10 local passes
• Ownership/reflection performance gate: 500 cycles within 2 seconds
• Warm cumulative focused runs: 58-61 ms locally
• git diff --check
• Remote validation runs only on this cumulative top to minimize CI and artifacts

Native GitHub stack #157

1. Scope content properties to their HTML interfaces #125 — HTML interface ownership (Scope meta and template content properties to their HTML interfaces #79)
2. Expose item on child collections #126 — current cumulative top — child collection item() (Expose item(index) on children and childNodes collections #80)

Merge both layers atomically through Stack #157 after the current-head related check passes.