Restore DigitalRoot legacy API compatibility

[krotname]

Motivation

• Fix a source/binary compatibility regression introduced by renaming the public kata method from digital_root(int) to digitalRoot(int) so existing callers and Codewars harnesses continue to work.

Description

• Add a deprecated compatibility wrapper public static int digital_root(int n) that delegates to the existing digitalRoot(int) implementation to preserve a single centralized behavior.
• Document the wrapper with Javadoc and mark it @Deprecated to encourage migration while retaining the old API.
• Extend the parameterized test shouldReduceNumberToSingleDigit to assert the legacy method returns the same results as digitalRoot(int) and digitalRootRecursiveStream(int) for the existing test cases.

Testing

• Ran unit tests with mvn -B -DskipTests=false test, and all tests passed (597 tests, 0 failures/errors).
• Ran full verification with mvn -B verify, and the build succeeded with coverage thresholds met and no Checkstyle/PMD/SpotBugs issues.
• The changes are minimal and purely API-compatible, with behavior delegated to the unchanged implementation so existing functionality is preserved.

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Codex Task

[krotname]

Закрываю как superseded by #116: изменения перенесены в одну проверенную ветку, все review findings учтены; локальный mvn verify прошёл полностью.