Specialize only the fixed unary case in the bytecode generator and let
all other argument counts keep using the generic Call instruction. This
keeps the builtin bytecode simple while still covering the common fast
path.
The asm interpreter handles int32 inputs directly, applies the ToUint16
mask in-place, and reuses the VM's cached ASCII single-character
strings when the result is 7-bit representable. Non-ASCII single code
unit results stay on the dedicated builtin path via a small helper, and
the dedicated slow path still handles the generic cases.
