perf: replace preview-based fixer resolution in pipeline queue

[omsherikar]

Summary

RefactronPipeline._find_fixer_name iterates every registered fixer and calls fixer.preview(issue, "x = 1\n") on dummy code to see if preview succeeds. This is O(fixers × issues) and may not reflect real file context, so orchestration work is noisy and slow for large issue lists.

Suggested direction

• Central registry: map IssueCategory / issue type → candidate fixers (or ask AutoFixEngine for the responsible fixer without dummy preview).
• Fallback to preview only when ambiguous.
• Cache per-issue-type resolution.

Acceptance

• No regression in which issues get a fixer assigned.
• Measurable reduction in queue time for large queues (benchmark or unit test with counter).

Code

• refactron/core/pipeline.py — _find_fixer_name, queue_issues
• refactron/autofix/engine.py (if API changes)