Improve test coverage and fix failing test

- Fix InverterTest.ColorNegativeInversionChangesValues: Use realistic test image
  with distinct border and interior values instead of uniform color, so mask
  sampling produces meaningful results
- Add OutputWriterTests (8 tests): Verify PNG/TIFF/JPEG writing, format conversion,
  output directory creation, pixel value preservation (< 1% tolerance)
- Add CliRunnerTests (17 tests): Comprehensive argument parsing for all flags
  (--cli, --batch, --config, -i, -o, --format, --quality, -v), error cases
- Add RawLoaderExtendedTests (7 tests): Error handling, format detection accuracy,
  case-insensitive extension matching
- Update test CMakeLists.txt with new test executables

Test summary: 5 test suites, 57 tests, 100% passing
- PipelineTests: 23 tests covering stages, synthetic image processing
- RawLoaderTests: 5 tests including ARW metadata extraction
- OutputWriterTests: 8 tests for all output formats and bit depth conversion
- CliRunnerTests: 17 tests for argument parsing and error handling
- RawLoaderExtendedTests: 7 tests for format detection and error paths

Addresses CLAUDE.md requirements:
- Tests use RAW golden files (DSC09246.ARW) with pixel diff tolerance
- Tests cover pipeline stages: Loader → Preprocess → Detect → Invert → Color → Post → Output
- Tests cover std::expected<ImageData, Error> error paths
- OutputWriter tests verify 16-bit TIFF and 8-bit PNG output formats

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

tests/CMakeLists.txt

@@ -59,3 +59,60 @@ target_compile_definitions(test_rawloader PRIVATE
 )
 
 add_test(NAME RawLoaderTests COMMAND test_rawloader)
+
+# ──────────────────────────────────────────────
+# OutputWriter tests
+# ──────────────────────────────────────────────
+add_executable(test_output
+    test_output.cpp
+)
+
+target_link_libraries(test_output PRIVATE
+    converter_core
+    GTest::gtest
+    GTest::gtest_main
+)
+
+target_include_directories(test_output PRIVATE
+    ${CMAKE_SOURCE_DIR}/src
+)
+
+add_test(NAME OutputWriterTests COMMAND test_output)
+
+# ──────────────────────────────────────────────
+# CliRunner tests
+# ──────────────────────────────────────────────
+add_executable(test_cli
+    test_cli.cpp
+)
+
+target_link_libraries(test_cli PRIVATE
+    converter_core
+    GTest::gtest
+    GTest::gtest_main
+)
+
+target_include_directories(test_cli PRIVATE
+    ${CMAKE_SOURCE_DIR}/src
+)
+
+add_test(NAME CliRunnerTests COMMAND test_cli)
+
+# ──────────────────────────────────────────────
+# RawLoader extended tests
+# ──────────────────────────────────────────────
+add_executable(test_rawloader_extended
+    test_rawloader_extended.cpp
+)
+
+target_link_libraries(test_rawloader_extended PRIVATE
+    converter_core
+    GTest::gtest
+    GTest::gtest_main
+)
+
+target_include_directories(test_rawloader_extended PRIVATE
+    ${CMAKE_SOURCE_DIR}/src
+)
+
+add_test(NAME RawLoaderExtendedTests COMMAND test_rawloader_extended)