refactor: reorganize lib/src into layered folders and split core widget

- Groups files by responsibility (`controller/`, `models/`, `ui/`,
  `widget/`).
- Splits the 500+ line `ScrollInfinity` file into widget config,
  state (pagination/fetch logic), and footer builders using `part`
  files while preserving private-member access.
- Mirrors the layout under `test/src/` and removes a dead empty test
  stub.

Author: dariomatias-dev

lib/scroll_infinity.dart

@@ -1,2 +1,3 @@
-export 'src/scroll_infinity.dart';
-export 'src/scroll_infinity_controller.dart' hide ScrollInfinityControllerState;
+export 'src/controller/scroll_infinity_controller.dart'
+    hide ScrollInfinityControllerState;
+export 'src/widget/scroll_infinity.dart';