Feat: add compound assignment and shift operators

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.

fluxc/src/ast.rs

@@ -11,6 +11,20 @@ pub enum UnaryOp {
     AddrOf, // `&`
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CompoundAssignOp {
+    Add,    // `+=`
+    Sub,    // `-=`
+    Mul,    // `*=`
+    Div,    // `/=`
+    Rem,    // `%=`
+    BitAnd, // `&=`
+    BitOr,  // `|=`
+    BitXor, // `^=`
+    Shl,    // `<<=`
+    Shr,    // `>>=`
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum BinaryOp {
     // Logical
@@ -33,6 +47,9 @@ pub enum BinaryOp {
     Mul, // `*`
     Div, // `/`
     Rem, // `%`
+    // Shift
+    Shl, // `<<`
+    Shr, // `>>`
     // Assignment (lowest precedence, right-associative)
     Assign, // `=`
 }
@@ -123,6 +140,13 @@ pub enum ExprKind {
         lhs: Box<Expr>,
         rhs: Box<Expr>,
     },
+    // Compound assignment: `lhs op= rhs`  (expands to `lhs = lhs op rhs`)
+    CompoundAssign {
+        op: CompoundAssignOp,
+        op_span: Span,
+        lhs: Box<Expr>,
+        rhs: Box<Expr>,
+    },
 
     // Postfix
     Field {