fix(sys): type maxsize as nativeint instead of int

Python's sys.maxsize is 2^63−1 on 64-bit systems, which overflows Fable's
Int32 (the target type for F# int). nativeint maps to Python's native int,
preserving the full value. Re-adds the maxsize positivity test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dbrattli

src/stdlib/Sys.fs

@@ -35,7 +35,7 @@ type IExports =
     /// Version information encoded as a single integer
     abstract hexversion: int
     /// Maximum value a variable of type int can take
-    abstract maxsize: int
+    abstract maxsize: nativeint
     /// Maximum Unicode code point value
     abstract maxunicode: int
     /// Module search path - list of directory names where Python looks for modules

test/TestSys.fs

@@ -11,6 +11,10 @@ let ``test sys.platform is non-empty string`` () =
 let ``test sys.version is non-empty string`` () =
     sys.version.Length > 0 |> equal true
 
+[<Fact>]
+let ``test sys.maxsize is positive`` () =
+    sys.maxsize > 0n |> equal true
+
 [<Fact>]
 let ``test sys.maxunicode is 1114111`` () =
     sys.maxunicode |> equal 1114111