Fix: add assignment as right-associative expression

`=` was missing from the Pratt table, causing `a = b;` to fail with
"expected `;`, found `=`". Assignment is now BinaryOp::Assign with
binding power (2, 2) — lowest precedence, right-associative — so
`a = b = c` parses as `a = (b = c)`.

fluxc/src/ast.rs

@@ -33,6 +33,8 @@ pub enum BinaryOp {
     Mul, // `*`
     Div, // `/`
     Rem, // `%`
+    // Assignment (lowest precedence, right-associative)
+    Assign, // `=`
 }
 
 // ── Types ──────────────────────────────────────────────────────────────────────