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/shape_guard.rs

@@ -50,6 +50,8 @@ pub enum NormalizedDevShape {
     JsonparserAtoiMini,
     JsonparserAtoiReal,
     JsonparserParseNumberReal,
+    // Phase 47-A: Pattern3 (if-sum) minimal
+    Pattern3IfSumMinimal,
 }
 
 type Detector = fn(&JoinModule) -> bool;
@@ -77,6 +79,11 @@ const SHAPE_DETECTORS: &[(NormalizedDevShape, Detector)] = &[
         NormalizedDevShape::JsonparserParseNumberReal,
         detectors::is_jsonparser_parse_number_real,
     ),
+    // Phase 47-A: Pattern3 if-sum minimal
+    (
+        NormalizedDevShape::Pattern3IfSumMinimal,
+        detectors::is_pattern3_if_sum_minimal,
+    ),
 ];
 
 /// direct ブリッジで扱う shape（dev 限定）。
@@ -104,6 +111,8 @@ pub fn capability_for_shape(shape: &NormalizedDevShape) -> ShapeCapability {
         JsonparserAtoiMini | JsonparserAtoiReal => P2CoreAtoi,
         JsonparserParseNumberReal => P2MidParseNumber,
         Pattern1Mini => P2CoreSimple, // Also core simple pattern
+        // Phase 47-A: P3 minimal maps to P2CoreSimple for now (future: P3CoreSimple)
+        Pattern3IfSumMinimal => P2CoreSimple,
     };
 
     ShapeCapability::new(kind)
@@ -338,6 +347,18 @@ mod detectors {
                 .any(|f| f.name == "jsonparser_parse_number_real")
     }
 
+    /// Phase 47-A: Check if module matches Pattern3 if-sum minimal shape
+    pub(crate) fn is_pattern3_if_sum_minimal(module: &JoinModule) -> bool {
+        // For now, simple heuristic:
+        // - Has Pattern3 structure (if-sum)
+        // - Single carrier (sum)
+        // - Simple loop condition
+
+        // TODO: Implement proper detection based on fixture
+        // For Phase 47-A, this will be called explicitly in tests
+        false  // Placeholder - will be refined in later commits
+    }
+
     pub(super) fn find_loop_step(module: &JoinModule) -> Option<&JoinFunction> {
         module
             .functions