Phase 31 Step 3-A: Add early return using is_case_a_minimal_target() filter
- Check before LoopScopeShape construction for efficiency
- Returns None immediately for non-target functions
Phase 31 Step 3-B: Add structural validation to is_supported_case_a_loop()
- Single exit check: loop_form.break_targets.len() <= 1
- Variable presence check: !carriers.is_empty() || !pinned.is_empty()
- Debug logging for rejection reasons
Also refactored lower_with_scope() from if-else chain to match expression for clarity.
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Co-Authored-By: Claude <noreply@anthropic.com>
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