Add reusable bounded integer inputs for JavaFX editors

fx/docs/integer-spinners.md

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+# Bounded integer inputs
+
+`de.neitzel.fx.control.IntegerSpinners.nonNegative(min, max, value)` creates an
+editable JavaFX spinner with inclusive nonnegative bounds. Construct and use it
+on the JavaFX application thread.
+
+Every valid typed number immediately updates the value property, so a form can
+save its model without waiting for Enter or a focus change. Non-digit input and
+values above the maximum are rejected. Empty input and intermediate numbers
+below the minimum retain the last committed value and revert when focus leaves
+the editor. Increment/decrement use the normal JavaFX spinner behavior.
+
+The factory rejects negative/inverted bounds and out-of-range initial values.
+Applications own their field labels, units, and domain-specific bounds.