feat(selfhost): add &T (shared ref) and &mut T (mutable ref) type syntax

- Add tekRef and tekMutRef to TypeExpr enum (ast.bux)
- Parse &T and &mut T in parserParseType, before *T handling (parser.bux)
- Map tekRef/tekMutRef to tyPointer in HIR lowering (hir_lower.bux)
- Both emit as T* in C backend (same as raw pointer)
- Borrow checker rejects write-through-pointer in @[Checked] functions
  for all pointer types (safe default)

src/ast.bux

@@ -26,6 +26,8 @@ const tekNamed: int = 0;
 const tekPath: int = 1;
 const tekSlice: int = 2;
 const tekPointer: int = 3;
+const tekRef: int = 7;        // &T — shared reference
+const tekMutRef: int = 8;     // &mut T — mutable reference
 const tekTuple: int = 4;
 const tekSelf: int = 5;
 const tekFunc: int = 6;