phase29ab(p1): pattern2 carrier binding policy + loopbodylocal fixture

src/mir/builder/control_flow/joinir/patterns/pattern2/README.md

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+# Pattern2 (Loop with Break) - JoinIR
+
+## Scope / Criteria
+- `loop(...) { ... break ... }` (break present, no continue/return)
+- break condition is normalized to "break when <cond> is true"
+- loop variable comes from header condition or loop(true) counter extraction
+
+## LoopBodyLocal promotion
+- SSOT entry: `pattern2::api::try_promote`
+- Supported: A-3 Trim / A-4 DigitPos (promote LoopBodyLocal to carrier)
+- ConditionOnly carriers are recalculated per iteration (no host binding)
+
+## Carrier binding rules (Pattern2)
+- `CarrierInit::FromHost` -> host binding required
+- `CarrierInit::BoolConst(_)` / `CarrierInit::LoopLocalZero` -> host binding is skipped
+- ConditionOnly carriers must not use `FromHost`
+
+## Out of scope
+- multiple breaks / continue / return in the loop body
+- reassigned LoopBodyLocal or ReadOnlySlot contract violations
+- break conditions with unsupported AST shapes
+
+## Fail-Fast policy
+- `PromoteDecision::Freeze` -> Err (missing implementation or contract violation)
+- JoinIR lowering/merge contract violations -> Err
+
+## `Ok(None)` meaning
+- not Pattern2 (extractor returns None)
+- promotion NotApplicable (router fallback)