ABI: implement private ordinary-object creation and primitive unboxing

Parents: #163, #164 Depends on: #172, #175

Implement the shared ordinary-object construction and JSON-style boxed-primitive unwrapping boundary.

Acceptance:

• Export exact JSC__JSValue__createEmptyObject, JSC__JSValue__createEmptyObjectWithNullPrototype, and JSC__JSValue__unwrapBoxedPrimitive signatures.
• Create fresh ordinary objects with the selected realm Object.prototype; treat initial capacity strictly as an unobservable allocation hint.
• Create fresh objects with a true null prototype.
• Unwrap Number, String, Boolean, and BigInt wrapper objects to exact JSC64 primitives, including int32 canonicalization and negative-zero preservation.
• Leave non-wrapper objects and already-primitive values unchanged.
• Reject invalid/foreign-context inputs safely.
• Cover exact signatures, prototype identity, freshness, wrapper kinds, and numeric boundaries in the compile-link-runtime fixture.
• Update both inventories and README/API documentation without changing public/extension counts.

Completed on main in 06ec44c; documentation followed in 1c4f5ec.

Evidence:

• all 3/3 exact symbols link through the pinned fixture;
• ordinary constructors are fresh and share the selected realm Object.prototype even when a large capacity hint is supplied;
• the null-prototype constructor returns a true null prototype;
• Number unwrapping covers int32 minimum/maximum, the first out-of-range double, negative zero, and NaN;
• String, Boolean, and BigInt wrappers return exact primitive values;
• ordinary objects and existing primitives retain identity/encoding;
• foreign-context wrapper input returns the empty sentinel;
• zig build test-home-private-abi reports 24/24 linked runtime symbols;
• audits report Home 26/432, Bun core 26/422, public 117, extensions 19.