ABI: implement private arrays and indexed access

Parents: #163, #164 Depends on: #180

Implement the shared private array construction and indexed-access boundary.

Acceptance:

• Export exact JSC__JSValue__createEmptyArray, JSC__JSValue__putIndex, JSC__JSValue__push, JSC__JSValue__getDirectIndex, and JSC__JSObject__getIndex signatures.
• Create fresh selected-realm arrays with exact logical length and holes, including zero and maximum u32 lengths.
• Perform direct index writes and pushes without invoking inherited setters, while preserving sparse gaps and array length.
• Return the empty JSC64 value for absent direct indices and preserve present undefined as a distinct non-empty value.
• Make observable indexed reads perform ToObject/prototype/getter semantics and publish thrown values through #180's VM pending-exception cell.
• Publish RangeError/OOM failures through the same empty-result iff pending-exception invariant.
• Reject invalid, foreign-context, and non-array mutation inputs safely.
• Cover exact signatures, holes, sparse writes, maximum lengths, inherited getters/setters, thrown getters, and sibling-realm exception visibility in the compile-link-runtime fixture.
• Update both inventories and README/API documentation without changing public/extension counts.

Implemented and documented on main.

Evidence:

• implementation: 7e92e161 (feat(abi): add private array index operations)
• published ABI/README evidence: f91d90ca
• focused private runtime fixture: 45/45 symbols linked and passed
• Home inventory: 47/432 implemented, 385 pending
• Bun core inventory: 47/422 implemented, 375 pending
• public/extension accounting remains 117/19

The runtime matrix covers holes versus present undefined, inherited getter visibility, inherited setter bypass, sibling-realm exception visibility, sparse growth, index/length u32 boundaries, RangeError publication, primitive ToObject, and foreign/invalid inputs.