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/uc14_audience_cross_platform.csv

@@ -0,0 +1,15 @@
+email,fb_id,google_click_id,platform,first_seen,last_seen
+audience1@example.com,fb_77881122,,facebook,2026-01-05,2026-04-20
+audience1@example.com,,gclid_AAA111,google_ads,2026-02-10,2026-04-18
+audience1@example.com,,,organic_form,2026-03-15,2026-03-15
+audience2@example.com,fb_99887766,,facebook,2026-02-01,2026-04-15
+audience2@example.com,,gclid_BBB222,google_ads,2026-02-20,2026-04-22
+old.email@example.com,fb_55443322,,facebook,2025-06-10,2025-12-01
+new.email@example.com,fb_55443322,,facebook,2026-01-15,2026-04-20
+,fb_11223344,,facebook,2026-03-01,2026-04-01
+,,gclid_CCC333,google_ads,2026-03-10,2026-04-10
+fbonly@example.com,fb_44556677,,facebook,2026-03-20,2026-04-20
+googleonly@example.com,,gclid_DDD444,google_ads,2026-03-25,2026-04-22
+organiconly@example.com,,,organic_form,2026-04-01,2026-04-01
+combined@example.com,fb_88990011,gclid_EEE555,manual_merge,2026-04-10,2026-04-25
+,,,sms_list,2026-04-05,2026-04-05