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)`.

GRAMMAR.ebnf

@@ -33,7 +33,23 @@ top_level_def = func_def
 (* Expressions                                                      *)
 (* ================================================================ *)
 
-expr = or_expr ;
+expr = assign_expr ;
+
+
+(* --- Assignment (lowest-precedence binary operator) ---           *)
+(*                                                                  *)
+(* Uses token `=`; right-associative via recursion.                 *)
+(* The optional form encodes at-most-one assignment target: chains  *)
+(* like `a = b = c` parse as `a = (b = c)` thanks to right          *)
+(* recursion.                                                       *)
+(*                                                                  *)
+(* LL(1): after or_expr, peek at next token.                        *)
+(*   "=" → consume and recurse into assign_expr                     *)
+(*   other → return the or_expr as-is                               *)
+(* "=" is not in FIRST(stmt), so expr_stmt can still be             *)
+(* distinguished from other statement kinds.                        *)
+
+assign_expr = or_expr , [ "=" , assign_expr ] ;
 
 
 (* --- Logical OR (lowest-precedence binary operator) ---           *)
@@ -165,7 +181,9 @@ arg_list = [ expr , { "," , expr } ] ;
 (* if_stmt and while_stmt use expr_ns for their condition.          *)
 (* All other expression positions use the full expr.                *)
 
-expr_ns = or_expr_ns ;
+expr_ns = assign_expr_ns ;
+
+assign_expr_ns = or_expr_ns , [ "=" , assign_expr_ns ] ;
 
 or_expr_ns  = and_expr_ns , { "or"  , and_expr_ns } ;
 and_expr_ns = bitor_expr_ns , { "and" , bitor_expr_ns } ;