Add Widgetbook setup with theme configuration, analysis options, and initial component use cases

widgetbook/lib/slider_m3e/INDEX-slider_m3e.md

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+# INDEX — slider_m3e
+
+This index lists all Widgetbook use cases generated for the slider_m3e package.
+
+## Components and Variants
+
+- SliderM3E
+  - default
+  - discrete
+  - disabled
+  - extremes_0_to_100
+  - negative_range
+  - min_equals_max
+
+- RangeSliderM3E
+  - default
+  - discrete
+  - disabled
+  - extremes_0_to_100
+  - negative_range
+  - min_equals_max
+
+Notes
+- All variants follow plan/guide.md rules with @UseCase annotations and required method signatures.
+- Knobs are provided for critical parameters (values, min, max, divisions) and key visuals (size, emphasis, shapeFamily, density, value indicator).
+- Callbacks print helpful messages to the console.
+- Range labels are included where useful; complex configurations are kept simple with TODOs where appropriate.