Parses a line of whitespace-separated f64 values, accepting between min
and max values (inclusive). Returns a vector of exactly max elements,
padding with values from defaults when fewer than max are present.
Hot-path: parse up to 5 whitespace-separated f64s into a stack buffer.
Returns count of tokens actually present (before padding).
Pads out[found..max] from defaults when found < max and found >= min.