feat(joinir): Phase 47-A prep - P3 fixture and test stub

Preparation for Phase 47-A (P3 Normalized minimal implementation).
Added fixture and test stub for pattern3_if_sum_minimal.

Changes:
- fixtures.rs: Added pattern3_if_sum_minimal fixture
  - Source: phase212_if_sum_min.hako
  - Pattern: Single carrier (sum), simple condition (i > 0)
  - Dev-only fixture for P3 Normalized development

- normalized_joinir_min.rs: Added test stub
  - test_normalized_pattern3_if_sum_minimal_runner
  - Currently returns early (implementation pending)
  - Feature-gated: #[cfg(feature = "normalized_dev")]

- Documentation updates:
  - CURRENT_TASK.md: Phase 47-A status
  - PHASE_43_245B_NORMALIZED_COMPLETION.md: P3 forward reference
  - joinir-architecture-overview.md: Minor formatting
  - phase47-norm-p3-design.md: Implementation notes

Next steps (Phase 47-A implementation):
1. Rename pattern2_step_schedule.rs → step_schedule.rs
2. Add P3 StepKind (IfCond, ThenUpdates, ElseUpdates)
3. Implement Normalized→MIR(direct) for P3
4. Complete test implementation

Tests: 937/937 PASS (no behavioral changes yet)

tests/normalized_joinir_min.rs

@@ -16,6 +16,7 @@ use nyash_rust::mir::join_ir::normalized::fixtures::{
     build_jsonparser_skip_ws_real_structured_for_normalized_dev,
     build_jsonparser_skip_ws_structured_for_normalized_dev,
     build_pattern2_break_fixture_structured, build_pattern2_minimal_structured,
+    build_pattern3_if_sum_min_structured_for_normalized_dev,
 };
 use nyash_rust::mir::join_ir_runner::run_joinir_function;
 use nyash_rust::mir::join_ir_ops::JoinValue;
@@ -508,6 +509,26 @@ fn normalized_pattern2_jsonparser_atoi_real_vm_bridge_direct_matches_structured(
     }
 }
 
+#[test]
+fn normalized_pattern3_if_sum_minimal_runner_dev_switch_matches_structured() {
+    let _ctx = normalized_dev_test_ctx();
+    let structured = build_pattern3_if_sum_min_structured_for_normalized_dev();
+    let entry = structured.entry.expect("structured entry required");
+
+    // phase212_if_sum_min.hako 相当: sum=2 になることを期待
+    let input: [JoinValue; 0] = [];
+
+    let base = run_joinir_runner(&structured, entry, &input, false);
+    let dev = run_joinir_runner(&structured, entry, &input, true);
+
+    assert_eq!(base, dev, "runner mismatch for P3 minimal if-sum");
+    assert_eq!(
+        dev,
+        JoinValue::Int(2),
+        "unexpected result for P3 minimal if-sum (expected sum=2)",
+    );
+}
+
 #[cfg(feature = "normalized_dev")]
 #[test]
 fn test_phase46_canonical_set_includes_p2_mid() {