grammar: add '<<' shift-left safely (Phase 1); map to AST/MIR/VM; keep '>>' as legacy ARROW

- Tokenizer: add SHIFT_LEFT for '<<' (before <= detection)
- Parser: introduce parse_shift() layer; left-associative; only '<<' handled in Phase 1
- AST: add BinaryOperator::Shl and display
- MIR builder: map Shl -> BinaryOp::Shl
- Interpreter: implement Shl in both execution paths (integer-only, type error otherwise)

No change to '>>' (legacy ARROW remains). No runtime gate yet for SHL as it does not collide with existing syntax.

src/ast.rs

@@ -333,6 +333,7 @@ pub enum BinaryOperator {
     Multiply,
     Divide,
     Modulo,
+    Shl,      // << shift-left (Phase 1)
     Equal,
     NotEqual,
     Less,
@@ -361,6 +362,7 @@ impl fmt::Display for BinaryOperator {
             BinaryOperator::Multiply => "*",
             BinaryOperator::Divide => "/",
             BinaryOperator::Modulo => "%",
+            BinaryOperator::Shl => "<<",
             BinaryOperator::Equal => "==",
             BinaryOperator::NotEqual => "!=",
             BinaryOperator::Less => "<",