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

widgetbook/lib/m3e_design/INDEX-m3e_design.md

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+# Widgetbook Index — m3e_design
+
+This package (m3e_design) provides the design system core for Material 3 Expressive: ThemeExtension(s), tokens (colors, typography, spacing, shapes, motion), and utilities. It does not expose standalone user-facing Widgets.
+
+Therefore, no component use cases are generated for this package. If downstream packages want to showcase tokens visually, they should do so in their own Widgetbook contexts using this design system.
+
+## Discovery (Components)
+
+- Scanned packages/m3e_design/lib for public Widgets (classes extending StatelessWidget/StatefulWidget).
+- Result: none found — only ThemeExtension, token data classes, and helpers.
+
+```
+[ ] Component inventory:
+    - [ ] (none)
+```
+
+## Summary Table
+
+| Component | Variants |
+|---|---|
+| — No direct widgets in this package — | — |
+
+## Notes
+
+- Themes are globally injected in consuming apps; no themes are provided here for use cases.
+- Optional previews (typography, palette, spacing, shapes, motion) could be created as internal demo widgets under widgetbook/ if desired. Not included to keep this package lean and avoid exporting demo-only code.
+
+_Last updated: 2025-10-25_