ABI: implement private ToObject and prototype queries

[chrisbbreuer]

Parents: #163, #164
Depends on: #175, #176

Implement the shared private ToObject and prototype-query boundary.

Acceptance:

• Export exact JSC__JSValue__toObject and JSC__JSValue__getPrototype signatures.
• Preserve identity for ordinary objects and perform exact realm-aware ToObject boxing for Number, Boolean, String, Symbol, and BigInt primitives.
• Reject null and undefined through the private null/empty failure boundary.
• Return exact ordinary, null, function, wrapper, and proxy-observed prototypes.
• Reject invalid and foreign-context values safely.
• Cover exact signatures, object identity, all primitive wrapper kinds, null prototypes, and proxy traps in the compile-link-runtime fixture.
• Update both inventories and README/API documentation without changing public/extension counts.

[chrisbbreuer]

Implemented and documented on main in dacec16 and a2024aa. Focused evidence: Home 31/432 with 29/29 runtime symbols, Bun core 31/422, and public/extension accounting unchanged at 117/19.