ABI: implement private ToNumber coercion

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

Implement the shared private full-ECMAScript ToNumber boundary.

Acceptance:

• Export the exact Bun__JSValue__toNumber signature.
• Preserve exact primitive conversion for undefined, null, booleans, int32/double values, and numeric/non-numeric strings.
• Run ordinary object ToPrimitive with number hint, including Symbol.toPrimitive, valueOf, and toString ordering.
• Throw TypeError for Symbol and BigInt inputs or primitive results.
• Return NaN and publish conversion throws through #180 VM pending exceptions, while ordinary numeric NaN leaves no exception pending.
• Preserve same-VM sibling values and reject foreign-VM or invalid values safely.
• Preserve an already pending first exception rather than replacing it.
• Cover exact signatures, primitive matrices, coercion order, thrown hooks, Symbol/BigInt, sibling/foreign ownership, and NaN/exception distinction 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: 3ae44959 (feat(abi): add private ToNumber coercion)
• published ABI/README evidence: c1293524
• focused private runtime fixture: 48/48 symbols linked and passed
• Home inventory: 50/432 implemented, 382 pending
• Bun core inventory: 50/422 implemented, 372 pending
• public/extension accounting remains 117/19
• public C and Bun inventory audits pass with zero drift

The fixture covers primitive conversions and signed zero, ordinary NaN without pending state, same-VM sibling strings, Symbol.toPrimitive and fallback hook order, thrown hooks, Symbol/BigInt TypeError, foreign-value rejection, and first-pending-exception preservation.