Update migration plan: Phase 3 complete

Document source file reorganization into logical directories:
- Core/ - Main public API
- Decoder/ - Internal decoder implementation
- Encoder/ - Internal encoder implementation
- Parsing/ - CSV parsing infrastructure
- Extensions/ - Parallel, streaming, backpressure
- Utilities/ - Helper utilities

All 181 tests pass. LotoFuel verified working.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: g-cqd

MIGRATION_PLAN.md

@@ -1,6 +1,6 @@
 # CSV Parsing Locale-Aware Migration Plan
 
-## Status: Phase 1 Complete, Phase 2 Complete, Phase 3 Planned
+## Status: Phase 1-3 Complete
 
 ## Overview
 
@@ -142,6 +142,24 @@ Split `CSVDecoderTests.swift` (2079 lines, 100+ tests) into focused suites:
   - All 181 tests pass
   - Total test files: 10 (8 decoder + 1 encoder + 1 locale-aware)
 
+### Entry 5: 2024-12-28
+- ✅ Phase 3 source file organization implemented:
+  - Reorganized flat source layout into logical directories:
+    ```
+    Sources/CSVCoder/
+    ├── Core/          - CSVDecoder, CSVEncoder, errors (4 files)
+    ├── Decoder/       - Row/SingleValue decoders (2 files)
+    ├── Encoder/       - Row/SingleValue encoders, writers (4 files)
+    ├── Parsing/       - Parser, SIMD scanner, streaming (5 files)
+    ├── Extensions/    - Parallel, streaming, backpressure (5 files)
+    ├── Utilities/     - Locale, macros, indexed codable (4 files)
+    └── CSVCoder.docc/ - Documentation
+    ```
+  - All 181 CSVCoder tests pass
+  - LotoFuel build succeeds
+  - 24 LotoFuelServices tests pass
+  - 794/795 LotoFuel tests pass (1 pre-existing failure unrelated to reorganization)
+
 ---
 
 ## Code Locations