feat: add documentation, Streamlit GUI, and full source tree

- Rewrite README.md with project overview, quick-start, and CLI summary
- Add docs/CLI-REFERENCE.md with full flag reference and 8 recipe sections
- Add docs/DEVELOPER.md with architecture, data flow, and extension guides
- Rewrite src/core/__init__.py with public API exports and module docstring
- Add Streamlit GUI (src/gui/) with file upload, advanced options, interactive
  match group review with side-by-side diff, and download buttons
- Add .gitignore, requirements.txt, all source code, tests, and sample data
- Add streamlit to requirements.txt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test-cases/uc04_orders_consolidated.csv

@@ -0,0 +1,15 @@
+source_order_id,source,order_date,customer_email,customer_name,total
+SHOP-1001,shopify,2026-04-01,kara.miller@gmail.com,Kara Miller,45.99
+AMZ-A789X,amazon,2026-04-03,kara.miller@gmail.com,K Miller,32.50
+MAN-2026-04,manual,2026-04-05,kara@miller-design.com,Kara Miller,120.00
+SHOP-1023,shopify,2026-04-08,tom.harris@example.com,Tom Harris,67.00
+MAN-2026-09,manual,2026-04-10,tom.haris@example.com,Tom Harris,55.00
+SHOP-1045,shopify,2026-04-12,jenny.l@example.com,Jenny Lee,89.00
+AMZ-B221Y,amazon,2026-04-15,JENNY.L@EXAMPLE.COM,Jenny L.,44.50
+SHOP-2001,shopify,2026-04-02,alex@example.com,Alex Park,33.00
+SHOP-2002,shopify,2026-04-02,amy@example.com,Amy Park,33.00
+SHOP-3001,shopify,2026-04-06,single1@example.com,Single Buyer One,78.00
+AMZ-C100Z,amazon,2026-04-07,single2@example.com,Single Buyer Two,92.00
+MAN-2026-12,manual,2026-04-09,single3@example.com,Single Buyer Three,150.00
+SHOP-4001,shopify,2026-04-11,sales@bigcorp.com,Mike Johnson,200.00
+SHOP-4012,shopify,2026-04-14,sales@bigcorp.com,Sarah Wright,175.00