ABI: implement private numeric DateInstance bridge

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

Implement the shared private numeric DateInstance boundary.

Acceptance:

• Export exact JSC__JSValue__dateInstanceFromNumber and JSC__JSValue__getUnixTimestamp signatures.
• Create a real selected-realm Date object while preserving the supplied internal double without JavaScript constructor TimeClip.
• Preserve fractional values, signed zero, NaN, infinities, and values outside the JavaScript TimeClip range.
• Return NaN for non-Date and invalid inputs; accept every live owned Date cell because this getter has no global/VM parameter.
• Cover exact signatures, Date.prototype identity, freshness, raw-double boundaries, and rejection cases in the compile-link-runtime fixture.
• Keep string parsing, UTC/local conversion, and ISO formatting pending until their separate timezone/buffer contracts are exact.
• Update both inventories and README/API documentation without changing public/extension counts.

Implemented and documented on main in 6bdec54 and f2e55b6. Focused evidence: Home 33/432 with 31/31 runtime symbols, Bun core 33/422, and public/extension accounting unchanged at 117/19.