feat(joinir): Phase 188.3-P3.3 - Pattern6 continuation generation + Call callee fix

Phase 3-3 完了: 4関数モデル JoinIR 生成
- nested_loop_minimal.rs (337行, 新規): 4関数モデル実装
  - main(): エントリーポイント
  - loop_step(i, sum): outer loop header
  - inner_step(j, i_outer, sum): inner loop (tail recursion)
  - k_inner_exit(i, sum): outer continuation after inner loop
  - k_exit(sum): 最終 exit
- pattern6_nested_minimal.rs: lowering pipeline 実装
  - boundary 構築 (continuation_func_ids 設定)
  - JoinIRConversionPipeline 呼び出し
- instruction_rewriter.rs: latch incoming 拡張
  - continuation→header 呼び出し対応

Call callee 修正:
- call_generator.rs: callee フィールドを Callee::Global に設定
- joinir_block_converter.rs: emit_call_pair 使用に統一

smoke test 追加:
- phase1883_nested_minimal_vm.sh (integration)

既知の問題 (次タスク):
- ValueId(104) undefined: PHI/merge 問題
- JoinIR 関数パラメータの MIR マッピングが不完全

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/pattern6_nested_minimal.rs

@@ -100,18 +100,112 @@ pub(crate) fn can_lower(_builder: &MirBuilder, ctx: &LoopPatternContext) -> bool
 
 /// Lower Pattern6 (NestedLoopMinimal) to MIR
 ///
-/// Phase 188.3: Full implementation with continuation generation
+/// Phase 188.3 P1: Full implementation with JoinIR pipeline
 pub(crate) fn lower(
-    _builder: &mut MirBuilder,
+    builder: &mut MirBuilder,
     ctx: &LoopPatternContext,
 ) -> Result<Option<ValueId>, String> {
+    use super::super::trace;
+
     // Phase 3-1: Extract inner loop AST (validate exactly 1 inner loop)
-    let inner_ast = extract_inner_loop_ast(ctx)?;
+    let _inner_ast = extract_inner_loop_ast(ctx)?;
 
     // Phase 3-2: Validate strict mode constraints (Fail-Fast)
-    validate_strict_mode(inner_ast, ctx)?;
+    validate_strict_mode(_inner_ast, ctx)?;
 
-    // Phase 3-3 stub - full implementation next
-    // TODO: Implement continuation generation (outer_step, inner_step, k_inner_exit)
-    Err("[Pattern6] Nested loop lowering not yet implemented (Phase 3-3 pending)".to_string())
+    // Phase 3-3: Full implementation with JoinIR pipeline
+    trace::trace().debug("pattern6", "Calling Pattern 6 nested loop minimal lowerer");
+
+    // Build preprocessing context - Pattern6 shares Pattern1's infrastructure
+    use super::pattern_pipeline::{build_pattern_context, PatternVariant};
+    let pattern_ctx = build_pattern_context(builder, ctx.condition, ctx.body, PatternVariant::Pattern1)?;
+
+    trace::trace().varmap("pattern6_start", &builder.variable_ctx.variable_map);
+
+    // Create JoinValueSpace for unified ValueId allocation
+    use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
+    let mut join_value_space = JoinValueSpace::new();
+
+    // Call Pattern 6 lowerer with preprocessed scope
+    use crate::mir::join_ir::lowering::nested_loop_minimal::lower_nested_loop_minimal;
+    let join_module = match lower_nested_loop_minimal(pattern_ctx.loop_scope, &mut join_value_space) {
+        Some(module) => module,
+        None => {
+            trace::trace().debug("pattern6", "Pattern 6 lowerer returned None");
+            return Ok(None);
+        }
+    };
+
+    // Create boundary from context
+    // Phase 188.3: Critical - must include continuation_func_ids to prevent merge misdetection
+    use crate::mir::join_ir::lowering::carrier_info::CarrierRole;
+    use crate::mir::join_ir::lowering::inline_boundary::LoopExitBinding;
+    use crate::mir::join_ir::lowering::join_value_space::PARAM_MIN;
+    use crate::mir::join_ir::lowering::JoinInlineBoundaryBuilder;
+    use crate::mir::join_ir::lowering::nested_loop_minimal::{INNER_STEP, K_INNER_EXIT};
+
+    // Extract k_exit's parameter ValueId from join_module
+    let k_exit_func = join_module.require_function("k_exit", "Pattern 6");
+    let sum_exit_value = k_exit_func
+        .params
+        .first()
+        .copied()
+        .expect("k_exit must have parameter for exit value (sum)");
+
+    // Phase 188.3: Extract variables from variable_map
+    let sum_var_id = builder
+        .variable_ctx
+        .variable_map
+        .get("sum")
+        .copied()
+        .ok_or_else(|| "[Pattern6] sum variable not found in variable_map".to_string())?;
+
+    let i_var_id = pattern_ctx.loop_var_id;
+
+    // Create exit binding for sum (the final return value)
+    let sum_exit_binding = LoopExitBinding {
+        carrier_name: "sum".to_string(),
+        join_exit_value: sum_exit_value,
+        host_slot: sum_var_id,
+        role: CarrierRole::LoopState,
+    };
+
+    // Phase 188.3 CRITICAL: Include continuation_func_ids to prevent merge from
+    // selecting inner_step as loop header
+    use std::collections::BTreeSet;
+    let continuation_funcs = BTreeSet::from([
+        "k_exit".to_string(),
+        K_INNER_EXIT.to_string(),
+        INNER_STEP.to_string(),
+    ]);
+
+    let boundary = JoinInlineBoundaryBuilder::new()
+        .with_inputs(
+            vec![ValueId(PARAM_MIN), ValueId(PARAM_MIN + 1)], // main(i0, sum0)
+            vec![i_var_id, sum_var_id],                       // host variables
+        )
+        .with_exit_bindings(vec![sum_exit_binding])
+        .with_loop_var_name(Some(pattern_ctx.loop_var_name.clone())) // Phase 188.3: Enables header PHI for 'i'
+        .with_continuation_funcs(continuation_funcs)
+        .build();
+
+    // Use JoinIRConversionPipeline for unified conversion flow
+    use super::conversion_pipeline::JoinIRConversionPipeline;
+    let _ = JoinIRConversionPipeline::execute(
+        builder,
+        join_module,
+        Some(&boundary),
+        "pattern6",
+        ctx.debug,
+    )?;
+
+    // Return Void (loops don't produce values)
+    let void_val = crate::mir::builder::emission::constant::emit_void(builder);
+
+    trace::trace().debug(
+        "pattern6",
+        &format!("Nested loop complete, returning Void {:?}", void_val),
+    );
+
+    Ok(Some(void_val))
 }