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/core_stringutils_test.cpp

@@ -0,0 +1,65 @@
+#include <gtest/gtest.h>
+
+#include <regex>
+
+#include "olive/core/util/stringutils.h"
+
+using namespace olive::core;
+
+TEST(CoreStringUtils, Split)
+{
+  auto result = StringUtils::split("a,b,c", ',');
+  ASSERT_EQ(result.size(), 3u);
+  EXPECT_EQ(result[0], "a");
+  EXPECT_EQ(result[1], "b");
+  EXPECT_EQ(result[2], "c");
+}
+
+TEST(CoreStringUtils, SplitRegex)
+{
+  auto result = StringUtils::split_regex("one:two;three", std::regex("(:)|(;)| "));
+  ASSERT_EQ(result.size(), 3u);
+  EXPECT_EQ(result[0], "one");
+  EXPECT_EQ(result[1], "two");
+  EXPECT_EQ(result[2], "three");
+}
+
+TEST(CoreStringUtils, ToInt)
+{
+  bool ok = false;
+  EXPECT_EQ(StringUtils::to_int("42", &ok), 42);
+  EXPECT_TRUE(ok);
+
+  EXPECT_EQ(StringUtils::to_int("-7", 10, &ok), -7);
+  EXPECT_TRUE(ok);
+
+  EXPECT_EQ(StringUtils::to_int("ff", 16, &ok), 255);
+  EXPECT_TRUE(ok);
+
+  EXPECT_EQ(StringUtils::to_int("abc", &ok), 0);
+  EXPECT_FALSE(ok);
+}
+
+TEST(CoreStringUtils, ToStringLeftpad)
+{
+  EXPECT_EQ(StringUtils::to_string_leftpad(5, 3), "005");
+  EXPECT_EQ(StringUtils::to_string_leftpad(123, 2), "123");
+  EXPECT_EQ(StringUtils::to_string_leftpad(7, 4, '*'), "***7");
+}
+
+TEST(CoreStringUtils, Format)
+{
+  EXPECT_EQ(StringUtils::format("Hello %s %d", "world", 42),
+            "Hello world 42");
+}
+
+TEST(CoreStringUtils, Trim)
+{
+  std::string s = "  hello world  ";
+  StringUtils::trim(s);
+  EXPECT_EQ(s, "hello world");
+
+  EXPECT_EQ(StringUtils::trimmed("\t\nvalue\t\n"), "value");
+  EXPECT_EQ(StringUtils::ltrimmed("  left"), "left");
+  EXPECT_EQ(StringUtils::rtrimmed("right  "), "right");
+}