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)

src/mir/join_ir/normalized/fixtures.rs

@@ -7,6 +7,7 @@ use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
 use crate::mir::join_ir::lowering::loop_update_analyzer::UpdateExpr;
 use crate::mir::join_ir::lowering::loop_with_break_minimal::lower_loop_with_break_minimal;
+use crate::mir::join_ir::lowering::loop_with_if_phi_if_sum::lower_if_sum_pattern;
 use crate::mir::join_ir::JoinModule;
 use crate::mir::{BasicBlockId, ValueId};
 use crate::{config::env::joinir_dev_enabled, config::env::joinir_test_debug_enabled};
@@ -210,6 +211,91 @@ pub fn build_jsonparser_atoi_real_structured_for_normalized_dev() -> JoinModule
     module
 }
 
+/// Pattern3 if-sum minimal ループ（phase212_if_sum_min.hako 相当）を Structured で組み立てるヘルパー。
+///
+/// Phase 47-A: P3 Normalized の最小ケース検証用（dev-only）。
+pub fn build_pattern3_if_sum_min_structured_for_normalized_dev() -> JoinModule {
+    fn var(name: &str) -> ASTNode {
+        ASTNode::Variable {
+            name: name.to_string(),
+            span: Span::unknown(),
+        }
+    }
+
+    fn int_lit(value: i64) -> ASTNode {
+        ASTNode::Literal {
+            value: LiteralValue::Integer(value),
+            span: Span::unknown(),
+        }
+    }
+
+    fn bin(op: BinaryOperator, left: ASTNode, right: ASTNode) -> ASTNode {
+        ASTNode::BinaryOp {
+            operator: op,
+            left: Box::new(left),
+            right: Box::new(right),
+            span: Span::unknown(),
+        }
+    }
+
+    fn assignment(target: ASTNode, value: ASTNode) -> ASTNode {
+        ASTNode::Assignment {
+            target: Box::new(target),
+            value: Box::new(value),
+            span: Span::unknown(),
+        }
+    }
+
+    // Minimal if-sum pattern:
+    // loop(i < 3) {
+    //   if (i > 0) { sum = sum + 1 } else { sum = sum + 0 }
+    //   i = i + 1
+    // }
+    let loop_condition = bin(BinaryOperator::Less, var("i"), int_lit(3));
+    let if_condition = bin(BinaryOperator::Greater, var("i"), int_lit(0));
+
+    let then_update = assignment(
+        var("sum"),
+        bin(BinaryOperator::Add, var("sum"), int_lit(1)),
+    );
+    let else_update = assignment(
+        var("sum"),
+        bin(BinaryOperator::Add, var("sum"), int_lit(0)),
+    );
+    let counter_update = assignment(
+        var("i"),
+        bin(BinaryOperator::Add, var("i"), int_lit(1)),
+    );
+
+    let if_stmt = ASTNode::If {
+        condition: Box::new(if_condition),
+        then_body: vec![then_update],
+        else_body: Some(vec![else_update]),
+        span: Span::unknown(),
+    };
+    let body = vec![if_stmt.clone(), counter_update];
+
+    let mut join_value_space = JoinValueSpace::new();
+    let mut cond_env = ConditionEnv::new();
+
+    // Phase 220-D: ConditionEnv には少なくともループ変数 i を登録しておく。
+    let i_id = join_value_space.alloc_param();
+    cond_env.insert("i".to_string(), i_id);
+
+    let (module, _meta) =
+        lower_if_sum_pattern(&loop_condition, &if_stmt, &body, &cond_env, &mut join_value_space)
+            .expect("if-sum lowering should succeed for minimal pattern3");
+
+    if joinir_dev_enabled() && joinir_test_debug_enabled() {
+        eprintln!(
+            "[joinir/normalized-dev] pattern3_if_sum_min structured module: {:#?}",
+            module
+        );
+    }
+
+    module
+}
+
 /// まとめて import したいとき用のプレリュード。
 pub mod prelude {
     pub use super::{
@@ -219,5 +305,6 @@ pub mod prelude {
         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,
     };
 }