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_digit_test.cpp

@@ -0,0 +1,23 @@
+#include <gtest/gtest.h>
+
+#include "common/digit.h"
+
+TEST(CommonDigit, SingleDigit)
+{
+  EXPECT_EQ(olive::GetDigitCount(0), 1);
+  EXPECT_EQ(olive::GetDigitCount(5), 1);
+  EXPECT_EQ(olive::GetDigitCount(-5), 1);
+}
+
+TEST(CommonDigit, MultipleDigits)
+{
+  EXPECT_EQ(olive::GetDigitCount(10), 2);
+  EXPECT_EQ(olive::GetDigitCount(999), 3);
+  EXPECT_EQ(olive::GetDigitCount(1000), 4);
+  EXPECT_EQ(olive::GetDigitCount(-12345), 5);
+}
+
+TEST(CommonDigit, LargeValue)
+{
+  EXPECT_EQ(olive::GetDigitCount(123456789012345LL), 15);
+}