ABI: implement private UTF-16 string inclusion

Parents: #163, #164 Depends on: #180, #183

Implement the shared private string inclusion boundary.

Acceptance:

• Export exact JSC__JSValue__stringIncludes signature.
• Apply full ToString to the receiver first and search value second, preserving observable hook order.
• Search by UTF-16 code units from position zero, including empty needles, astral characters, and isolated/high/low surrogate substrings.
• Return false and publish either coercion failure through #180, preserving thrown value identity and the first pending exception.
• Preserve same-VM sibling values and reject foreign-VM values safely.
• Cover exact signature, primitive/object coercion, order, UTF-16 edge cases, thrown hooks, sibling/foreign ownership, and pending-state behavior 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: 0ead0c65
• published README/API evidence: 18c64cb5
• focused private runtime fixture: 51/51 symbols linked and passed
• Home inventory: 53/432 implemented, 379 pending
• Bun core inventory: 53/422 implemented, 369 pending
• public/extension accounting remains 117/19; public C and Bun audits pass

The fixture covers receiver-then-search ToString order, primitive/object and same-VM sibling coercion, empty/ordinary substrings, astral values and each surrogate half, thrown-value identity for either coercion, foreign ownership, and pending-state behavior.