🧹 Refactor block deduplication overlap logic

[Jandir]

🎯 What: The list overlap and comparison logic inside lexis._get_deduplication_start_index has been simplified and refactored. The previous complex sequence of if-statements that looked for identical starts/ends or full containment has been replaced by a clean algorithm that iterates backward from the maximum possible overlap down to zero.

💡 Why: The previous logic was fragile and didn't correctly calculate overlaps when two identically sized lists had partial inner overlaps greater than 1 item, or when comparing completely identical lists. Replacing it with a deterministic suffix-to-prefix check vastly improves correctness, maintainability, and removes edge cases where rolling subtitle chunks might get misaligned.

✅ Verification: I added explicitly defined unit tests to tests/test_lexis.py to cover both partial overlaps of variable size and full duplicate block checks. I ran the pytest suite on lexis.py, and all test cases, including the new edge cases, pass correctly.

✨ Result: A single, easy-to-read mathematical loop gracefully covers all overlap scenarios, yielding a much more robust subtitle deduplication pipeline.

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PR created automatically by Jules for task 12091645625212671855 started by @Jandir

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