perf: optimize conflict resolution algorithm in AnonymizerEngine

[Ray0907]

Change Description

• Replace O(n) list.remove() with O(1) set operations for tracking active indices
• Use pop() instead of remove() in whitespace merge since prev_result is always last
• Convert list comprehension to generator expression for short-circuit evaluation
• Pre-compile regex pattern to avoid repeated compilation

Issue reference

Fixes #XX

Checklist

• [x] I have reviewed the contribution guidelines
• [x] I have signed the CLA (if required)
• [x] My code includes unit tests
• [x] All unit tests and lint checks pass locally
• [ ] My PR contains documentation updates / additions if required

[Ray0907]

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