test: expand gtest coverage for common/core utilities

Add unit tests for:
- CommandLineParser, Debug handler, Decibel, Digit, JobTime, FFmpegUtils
- Core StringUtils, Timecode, TimeRange, Bezier, Color
- Extend Frame tests to cover pixel access, interlace, conversion

Coverage improvements (app + ext/core):
- commandlineparser.cpp: 0% -> 93%
- debug.cpp: 0% -> 85%
- decibel.h: 25% -> 88%
- digit.h: 0% -> 100%
- ffmpegutils.cpp: 29% -> 90%
- frame.cpp: 53% -> 97%
- bezier.cpp: 16% -> 98%
- color.cpp: 13% -> 71%
- Overall line coverage: 17% -> 19%

tests/gtest/common_decibel_test.cpp

@@ -0,0 +1,52 @@
+#include <gtest/gtest.h>
+
+#include "common/decibel.h"
+
+TEST(CommonDecibel, FromLinearZeroReturnsMinimum)
+{
+  EXPECT_DOUBLE_EQ(olive::Decibel::fromLinear(0.0), olive::Decibel::MINIMUM);
+}
+
+TEST(CommonDecibel, FromLinearOneReturnsZero)
+{
+  EXPECT_DOUBLE_EQ(olive::Decibel::fromLinear(1.0), 0.0);
+}
+
+TEST(CommonDecibel, FromLinearTenReturnsTwenty)
+{
+  EXPECT_DOUBLE_EQ(olive::Decibel::fromLinear(10.0), 20.0);
+}
+
+TEST(CommonDecibel, ToLinearZeroReturnsOne)
+{
+  EXPECT_DOUBLE_EQ(olive::Decibel::toLinear(0.0), 1.0);
+}
+
+TEST(CommonDecibel, ToLinearMinimumReturnsZero)
+{
+  EXPECT_DOUBLE_EQ(olive::Decibel::toLinear(olive::Decibel::MINIMUM), 0.0);
+}
+
+TEST(CommonDecibel, ToLinearTwentyReturnsTen)
+{
+  EXPECT_DOUBLE_EQ(olive::Decibel::toLinear(20.0), 10.0);
+}
+
+TEST(CommonDecibel, FromLogarithmicAtEdges)
+{
+  EXPECT_DOUBLE_EQ(olive::Decibel::fromLogarithmic(0.0),
+                   olive::Decibel::MINIMUM);
+  EXPECT_DOUBLE_EQ(olive::Decibel::fromLogarithmic(1.0), 0.0);
+}
+
+TEST(CommonDecibel, ToLogarithmicAtEdges)
+{
+  EXPECT_DOUBLE_EQ(olive::Decibel::toLogarithmic(0.0), 1.0);
+}
+
+TEST(CommonDecibel, LinearLogarithmicRoundTrip)
+{
+  EXPECT_NEAR(olive::Decibel::LogarithmicToLinear(
+                  olive::Decibel::LinearToLogarithmic(0.5)),
+              0.5, 1e-6);
+}