Add a sample Power BI `.pbix` that loads Spindle retail output

[sqllocks]

One of the highest-leverage demo assets we're missing: a ready-to-open .pbix file that loads a Spindle-generated retail dataset.

What to do

1. Run:

   pip install sqllocks-spindle pyarrow
   spindle generate retail --scale large --seed 42 --output ./demo-data/ --format parquet

   (Note: pyarrow is required for --format parquet — see Issue Parquet output requires pip install pyarrow (not auto-installed) #6.)

2. Open Power BI Desktop, Get Data → Folder → ./demo-data/, load all tables

3. Build a small star-schema model (Customer + Product + Date dims, Order facts)

4. Add 3–4 visuals that show off realistic patterns:

   • Seasonal sales line chart (Spindle generates real seasonality)
   • Geographic map (Spindle ships real US lat/lng addresses)
   • Pareto / top-customers chart
   • Customer LTV histogram

5. Save the .pbix and add it to examples/powerbi/spindle-retail-demo.pbix

6. Add a short examples/powerbi/README.md with screenshots

Why this is a good first issue

• No code required — just Power BI Desktop
• Showcases the "data looks like production data" claim from the README
• Reusable in launch posts and tutorials

If you don't have Power BI Desktop, a .pbit template (without data) is equally welcome.