Task 432: Extract and Test deduplicateRollingText from extract-dialogue.ts

Status: DONE

Summary

Extract deduplicateRollingText from extract-dialogue.ts (CLI script with bare main() call) into dialogue-extraction.ts (already testable module) and add comprehensive tests. This is core subtitle processing logic for handling YouTube auto-generated rolling subtitle deduplication.

Rationale

• Core text processing function with edge-case-prone logic (multi-line rolling text detection)
• Currently untestable because it's in a script module that executes on import
• dialogue-extraction.ts already exports functions and has tests