feat: add borrow test example, fix borrow &mut parsing order

- examples/borrow.bux: working borrow expression example
- tests/borrow_test.nim: add 4 new tests for borrow & @[Shared]
- parser.nim: fix borrow parsing order (& before mut)
- Build verified: example compiles and runs

bootstrap/hir_lower.nim

@@ -449,6 +449,8 @@ proc resolveExprType(ctx: var LowerCtx, expr: Expr): Type =
       if last.kind == skExpr:
         return ctx.resolveExprType(last.stmtExpr)
     return makeVoid()
+  of ekBorrow:
+    return ctx.resolveExprType(expr.exprBorrowOperand)
   else: return makeUnknown()
 
 proc extractGenericStructInfo(ctx: LowerCtx, te: TypeExpr): tuple[baseName: string, typeArgs: seq[TypeExpr]] =
@@ -896,6 +898,11 @@ proc lowerExpr(ctx: var LowerCtx, expr: Expr): HirNode =
     let lowered = ctx.lowerExpr(expr.exprAwaitOperand)
     return hirCall("bux_async_await", @[lowered], makePointer(makeVoid()), loc)
 
+  of ekBorrow:
+    # borrow &mut expr — lowered to the operand directly (borrow is a no-op in HIR)
+    # The borrow checker validates before lowering
+    return ctx.lowerExpr(expr.exprBorrowOperand)
+
   else:
     return HirNode(kind: hLit, litToken: Token(kind: tkIntLiteral, text: "0", loc: loc),
                    typ: makeVoid(), loc: loc)