BigInt primitive support

Date: 2026-04-18 Area: runtime

BigInt primitive type (ES2020). Previously excluded from scope because GocciaScript's use cases did not demand arbitrary-precision integers. Reconsidered because (1) test262 tests for the Temporal API require BigInt as a prerequisite — nanosecond-precision epochs (Temporal.Instant.epochNanoseconds) return BigInt values, and (2) several Temporal internals are simplified with BigInt primitives available rather than workarounds. The implementation is self-contained (TBigInteger record in BigInteger.pas, TGocciaBigIntValue wrapping it) with no impact on existing Number paths. built-ins.md § BigInt. language-tables.md.