Prevent regex metacharacter crashes in CamelCase.toCamelCase

[krotname]

Motivation

• toCamelCase used String.replaceFirst with the first character as a regex/replacement, which can throw PatternSyntaxException or IllegalArgumentException for inputs starting with metacharacters and crashes on null input.

Description

• Add a null/empty guard in kyu6.CamelCase.toCamelCase to return an empty string for null or empty input.
• Replace the regex-based replaceFirst call with literal substring concatenation: s.substring(0,1).toUpperCase(Locale.ROOT) + s.substring(1) to avoid regex/replacement parsing of the first character.
• Add regression tests in src/test/java/kyu6/CamelCaseTest.java that assert correct behavior for inputs containing regex/replacement metacharacters and for null input.

Testing

• Ran the CamelCase unit test: mvn -q -Dtest=CamelCaseTest test and it passed.
• Ran the full test suite: mvn -q test and it completed successfully for this change.
• Ran git diff --check to ensure no whitespace or patch issues (no problems reported).

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Codex Task

[krotname]

Superseded by the validated combined merge in #115: #115