- Add CarrierInit enum (FromHost/BoolConst) for explicit initialization policy
- LoopHeaderPhiBuilder generates Const instruction for BoolConst carriers
- merge/mod.rs uses carrier_info to include ALL carriers in header PHI
- Pattern 2/4 pass carrier_info to boundary builder
- ConditionOnly carriers (is_digit_pos) now included in header PHI
Remaining: latch incoming for ConditionOnly carriers (Phase 228-8)
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>