chore(deps): bump typescript to 6.0 and vitest to 4.1.9

Dev-tooling only — no change to the published package or emitted
.d.ts files. TypeScript 6.0 now requires an explicit rootDir for
declaration emit (error TS5011), so set rootDir: "src" in tsconfig;
output layout is unchanged. All gates pass (lint, format, typecheck,
40/40 tests, build).

Author: tkirda-bison