Idiomatic TypeScript types

Express data and contracts precisely in the TypeScript type system, and configure the compiler so the types actually hold.

Targets TypeScript 5.9 (current stable, Q1 2026): redesigned tsc --init defaults (module: nodenext, target: esnext, moduleDetection: force, strict on), import defer for deferred module evaluation, expandable editor hovers. The TypeScript 7 native compiler (tsgo, the Go port) is in preview as @typescript/native-preview and type-checks roughly 10x faster (VS Code's 1.5M LOC: 89s -> 8.74s) — adopt it for fast local checks; stable 7.0 lands early 2026.

Boundary: language vs runtime vs framework