Form Mode Is Two-Way: Why So Many URL Encoders Break on a Literal `+`

WHATWG's x-www-form-urlencoded serializer maps space → '+' AND '+' → %2B in the same breath. Drop the second half and round-trip silently corrupts every input with a literal plus sign — the C++ language name, a phone country code, an arithmetic expression. Most ad-supported URL encoders ship only half of the rule.