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/uc15_suppression_combined.csv

@@ -0,0 +1,18 @@
+email,suppression_reason,suppressed_date,source_list
+supp1@example.com,unsubscribe,2025-08-15,list_main
+supp1@example.com,hard_bounce,2025-09-20,list_promo
+supp1@example.com,spam_complaint,2025-10-01,list_main
+supp2@example.com,unsubscribe,2026-01-10,list_main
+supp2@example.com,unsubscribe,2026-01-10,list_promo
+supp2@example.com,unsubscribe,2026-01-10,list_newsletter
+Supp3@Example.com,unsubscribe,2025-12-05,list_main
+supp3@example.com,hard_bounce,2026-02-15,list_promo
+bounced@gmail.com,hard_bounce,2026-03-01,list_main
+b.o.u.n.c.e.d@gmail.com,hard_bounce,2026-03-01,list_promo
+complainer@example.com,spam_complaint,2026-02-20,list_main
+soft@example.com,soft_bounce_x3,2026-03-15,list_main
+solo_unsub@example.com,unsubscribe,2026-04-01,list_main
+solo_bounce@example.com,hard_bounce,2026-04-05,list_promo
+solo_complaint@example.com,spam_complaint,2026-04-10,list_main
+padded@example.com   ,unsubscribe,2026-04-12,list_main
+padded@example.com,unsubscribe,2026-04-12,list_promo