feat: replace public byteAt with private byteAt_ and add O(1) bytes.count

Remove the public `byteAt(_)` method from String and its ByteSequence wrapper, renaming it to the private `byteAt_(_)` to eliminate redundancy with `.bytes[_]`. Add a native `byteCount_` primitive on String and expose it as `count` on StringByteSequence, enabling O(1) byte length queries. Update all test files to use the new `.bytes` subscript API and add a dedicated `count.wren` test for the new functionality.

builtin/core.wren

@@ -137,9 +137,11 @@ class StringByteSequence is Sequence {
     _string = string
   }
 
-  [index] { _string.byteAt(index) }
+  [index] { _string.byteAt_(index) }
   iterate(iterator) { _string.iterateByte_(iterator) }
-  iteratorValue(iterator) { _string.byteAt(iterator) }
+  iteratorValue(iterator) { _string.byteAt_(iterator) }
+
+  count { _string.byteCount_ }
 }
 
 class List is Sequence {