fix(llvm): Phase 131-8 - ExternCall argument handling 修正

Phase 131-8: ExternCall が PHI 値を正しく受け取るように修正

問題:
- ExternCall が resolve_ptr() を使用
- PHI 値は local vmap にない → null を返す
- 結果: console.log が常に 0 を出力

修正:
- src/llvm_py/instructions/externcall.py:
  - resolve_ptr() → resolve_i64_strict() に統一
  - 他の命令（binop, compare, copy）と同じ resolver を使用

テスト結果:
- Before: 0,0,0
- After: 0,01,011 (PHI 値が正しく渡る！)
- Case A: ✅ 退行なし (exit code 42)

新規発見（スコープ外）:
- MIR 型推論バグ: %3 が String 型として扱われる
- 出力が 0,1,2 ではなく 0,01,011（文字列連結）
- → Rust frontend の問題（Phase 131-9 候補）

箱化モジュール化:
- ✅ resolve_i64_strict() に統一
- ✅ Value resolution vs Type conversion の責務分離
- ✅ 構造的解決

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

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

src/llvm_py/instructions/externcall.py

@@ -7,6 +7,7 @@ import llvmlite.ir as ir
 from typing import Dict, List, Optional, Any
 from instructions.safepoint import insert_automatic_safepoint
 from instructions.extern_normalize import normalize_extern_name
+from utils.values import resolve_i64_strict
 
 def lower_externcall(
     builder: ir.IRBuilder,
@@ -103,20 +104,19 @@ def lower_externcall(
     call_args: List[ir.Value] = []
     for i, arg_id in enumerate(args):
         orig_arg_id = arg_id
-        # Prefer resolver/ctx
+        # ALWAYS resolve as i64 first (handles PHI values correctly)
+        # Type coercion to pointer happens below if needed
         aval = None
         if resolver is not None and preds is not None and block_end_values is not None and bb_map is not None:
             try:
-                if len(func.args) > i and isinstance(func.args[i].type, ir.PointerType):
-                    aval = resolver.resolve_ptr(arg_id, builder.block, preds, block_end_values, vmap)
-                else:
-                    aval = resolver.resolve_i64(arg_id, builder.block, preds, block_end_values, vmap, bb_map)
+                # Use strict resolver (handles PHI values correctly, same as binop/compare/copy)
+                aval = resolve_i64_strict(resolver, arg_id, builder.block, preds, block_end_values, vmap, bb_map)
             except Exception:
                 aval = None
         if aval is None:
             aval = vmap.get(arg_id)
         if aval is None:
-            # Default guess
+            # Fallback (should rarely hit with resolve_i64_strict)
             aval = ir.Constant(i64, 0)
 
         # If function prototype is known, coerce to expected type