docs: update IMPLEMENTATION.md with progress

Author: Developer

docs/IMPLEMENTATION.md

@@ -282,17 +282,17 @@
     - [x] Real EXIF data extraction (WASM findEXIFOffset function) ✅
     - [x] Real histogram generation (WASM calculateHistogram function) ✅
     - [x] Implement actual image format validation ✅
-  - [ ] Canvas implementation cleanup
-    - [ ] Remove test-only mock color returns (lines 93-98)
-    - [ ] Clean up Node.js test branches
-    - [ ] Optimize dominant color extraction algorithm
+  - [x] Canvas implementation cleanup ✅
+    - [x] Remove test-only mock color returns (lines 93-98) ✅
+    - [x] Clean up Node.js test branches ✅
+    - [x] Optimize dominant color extraction algorithm (k-means clustering) ✅
   - [x] Performance optimizations ✅
     - [x] Implement WASM streaming compilation ✅
     - [x] Add WebAssembly.compileStreaming support ✅
     - [x] Optimize memory usage for large images ✅
     - [x] Implement image sampling strategies (limits to 50MB) ✅
   - [ ] Testing and validation
-    - [ ] Remove test-only utilities (forceError flag)
+    - [x] Remove test-only utilities (forceError flag) ✅
     - [ ] Add real image test fixtures
     - [ ] Validate against various image formats
     - [ ] Browser compatibility testing