typescript-clean-codelisted

# TypeScript clean code Generic clean-code sense is assumed. Apply these TypeScript-specific rules. See `references/typescript-patterns.md` for worked examples. ## Types - **Make illegal states unrepresentable.** Prefer a discriminated union to optional fields plus a flag. - **Prefer `unknown` to `any`.** Narrow unknown input. Isolate and explain `any` required by untyped interop, compatibility work or deliberate escape hatches. - **Do not launder types with assertions.** Narrow with guards, `in`, `typeof` or `instanceof`, or validate boundaries. Avoid `as` and `as unknown as`; `as const` and `satisfies` are fine. - **Enforce exhaustiveness with `never`.** Use `default: assertNever(x)` for union switches. - **Derive, do not duplicate.** Use `z.infer`, `keyof`, `typeof`, mapped types, `Pick` or `Omit`; keep one source for each shape. - Use `type` for unions/functions/mapped types and follow the repository's established `type`/`interface` convention for object shapes. - **Brand primitives that must not be interchanged**, such as `UserId` and `OrderId`. ## tsconfig - Target `strict`, `noUncheckedIndexedAccess` (indexed access can be undefined), `exactOptionalPropertyTypes`, `noImplicitOverride`, `noFallthroughCasesInSwitch` and `useUnknownInCatchVariables`. For an existing codebase, stage flags with an explicit migration and no silent public-type changes. - Run `tsc --noEmit` in CI. ## Errors & null - **`catch` binds `unknown`.** Narrow before acce