0.21.0

• breaking tf: rename fs/dev/tf → fs/emergent-testing; the public exports and the external-runner entry import change accordingly #923
• asserts: extract assert, assertEq, todo, and Assert<T> from fs/dev/module.f.ts into a standalone fs/asserts/module.f.ts #923
• types/nullable: add fromUndefined(v) — names the JS-host ↔ FunctionalScript undefined→null boundary in one helper #919
• effects/node: add errorExit(s) — the canonical "write an error line to stderr, yield exit code 1" NodeOp program; replaces seven inline copies #917