feat: add examples/demo.csv and examples/demo.improv

40-row CSV with obviously-fake sales data (fictional companies like
Acme Corp, Wonka Industries, Cyberdyne Systems). demo.improv generated
via headless import with Profit formula and a default view showing
Region+Product on rows, Date_Month+Measure on columns. Added the
import command to the README quick-start section.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -33,6 +33,22 @@ Or import your own CSV:
 ./result/bin/improvise import path/to/data.csv
 ```
 
+The included `examples/demo.improv` was generated from `examples/demo.csv`:
+
+```sh
+improvise import examples/demo.csv \
+  --no-wizard \
+  --category Region --category Product --category Customer \
+  --measure Revenue --measure Cost \
+  --time Date --extract Date:Month \
+  --axis Region:row --axis Product:row \
+  --axis Date_Month:column --axis Measure:column \
+  --axis Customer:page --axis Date:none \
+  --formula "Profit = Revenue - Cost" \
+  --name "Acme Sales Demo" \
+  -o examples/demo.improv
+```
+
 ## Key bindings to try first
 
 | Key | Action |