Compound assignment: +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>= Shift: <<, >> Each compound assignment token parses at the same precedence as `=` (right-associative, lowest) and produces ExprKind::CompoundAssign. Shifts parse between additive and multiplicative precedence. GRAMMAR.ebnf and SYNTAX.md updated accordingly.