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/lowering/mod.rs

@@ -61,7 +61,7 @@ pub(crate) mod loop_view_builder; // Phase 33-23: Loop lowering dispatch
 pub mod loop_with_break_minimal; // Phase 188-Impl-2: Pattern 2 minimal lowerer
 pub mod loop_with_continue_minimal;
 pub mod method_call_lowerer; // Phase 224-B: MethodCall lowering (metadata-driven)
-pub(crate) mod pattern2_step_schedule; // Phase 39: Pattern2 evaluation order scheduler
+pub(crate) mod step_schedule; // Phase 47-A: Generic step scheduler for P2/P3 (renamed from pattern2_step_schedule)
 pub mod method_return_hint; // Phase 83: P3-D 既知メソッド戻り値型推論箱
 pub mod scope_manager; // Phase 231: Unified variable scope management // Phase 195: Pattern 4 minimal lowerer
                        // Phase 242-EX-A: loop_with_if_phi_minimal removed - replaced by loop_with_if_phi_if_sum