Exact decimal arithmetic for financial and precision-sensitive applications. Software implementation in the runtime (WebAssembly has no native decimal floating-point).

Split from #62 (which originally covered Map, Set, and Decimal). Map and Set are being implemented in PRs #332 and a follow-up. Decimal is independent of the Map/JSON/HTTP critical path and can be scheduled separately.

Spec reference: Chapter 9, Section 9.7.2