feat(joinir): Phase 47-A-IMPL - P3 Normalized infrastructure

Implement Pattern3 (if-sum) Normalized infrastructure, extending existing P2
StepSchedule and ShapeGuard systems.

Key changes:

1. StepSchedule generalization (P2 → P2/P3):
   - Renamed: pattern2_step_schedule.rs → step_schedule.rs
   - Extended StepKind enum with P3 variants:
     - IfCond (if condition in body)
     - ThenUpdates (carrier updates in then branch)
     - ElseUpdates (carrier updates in else branch)
   - Added pattern3_if_sum_schedule() function
   - Added unit test: test_pattern3_if_sum_schedule()
   - Updated module references (mod.rs, loop_with_break_minimal.rs)

2. ShapeGuard extension:
   - Added Pattern3IfSumMinimal variant to NormalizedDevShape
   - Added is_pattern3_if_sum_minimal() detector (placeholder)
   - Updated shape detector table
   - Extended capability_for_shape() mapping

3. Bridge integration:
   - bridge.rs: Added P3 shape handling in normalize_for_shape()
   - normalized.rs: Added P3 roundtrip match (uses P2 temporarily)

4. P2/P3 separation:
   - loop_with_break_minimal.rs: Added panic for P3 steps in P2 lowering
   - Clear boundary enforcement (P2 lowerer rejects P3 steps)

5. Documentation:
   - CURRENT_TASK.md: Phase 47-A-IMPL status
   - phase47-norm-p3-design.md: Implementation status section

Benefits:
- Reuses 90% of P2 infrastructure (ConditionEnv, CarrierInfo, ExitLine)
- Clean P2/P3 separation via StepKind
- Pure additive changes (no P2 behavioral changes)
- Ready for Phase 47-A-LOWERING (full P3 Normalized implementation)

Tests: 938/938 PASS (+1 from step_schedule unit test)
- All existing P1/P2 tests pass (no regressions)
- P3 test uses Structured path temporarily (proper lowering in next phase)

Next phase: Implement full P3 Normalized→MIR(direct) lowering

src/mir/join_ir/normalized.rs

@@ -905,6 +905,11 @@ pub(crate) fn normalized_dev_roundtrip_structured(
                 let norm = normalize_pattern2_minimal(module);
                 normalized_pattern2_to_structured(&norm)
             })),
+            // Phase 47-A: P3 minimal uses P2 normalization for now
+            NormalizedDevShape::Pattern3IfSumMinimal => catch_unwind(AssertUnwindSafe(|| {
+                let norm = normalize_pattern2_minimal(module);
+                normalized_pattern2_to_structured(&norm)
+            })),
         };
 
         match attempt {