feat(llvm): Phase 131-15 - print/concat segfault 根治修正

## P1-1/P1-2: TypeFacts 優先化
- binop.py: operand facts が dst_type ヒントより優先
- mir_json_emit.rs: Unknown 時に dst_type を出さない
- function_lower.py: value_types を metadata から読み込み

## P2: handle concat 統一（根治）
- print シグネチャ修正: i64(i64) → void(i8*)
- Mixed concat を handle ベースに統一:
  - concat_si/concat_is → concat_hh
  - box.from_i64 で integer を handle 化
  - Everything is Box 哲学に統一
- legacy 関数は互換性のために保持

## 結果
- ✅ print("Result: " + 3) → Result: 3
- ✅ segfault 解消
- ✅ Everything is Box 統一

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

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

crates/nyash_kernel/src/lib.rs

@@ -119,7 +119,6 @@ pub extern "C" fn nyash_string_concat_hh_export(a_h: i64, b_h: i64) -> i64 {
     nyash_rust::runtime::global_hooks::gc_alloc(s.len() as u64);
     let arc: std::sync::Arc<dyn NyashBox> = std::sync::Arc::new(StringBox::new(s));
     let h = handles::to_handle_arc(arc) as i64;
-    eprintln!("[TRACE] concat_hh -> {}", h);
     h
 }
 
@@ -239,7 +238,6 @@ pub extern "C" fn nyash_box_from_i8_string(ptr: *const i8) -> i64 {
     let arc: std::sync::Arc<dyn NyashBox> = std::sync::Arc::new(StringBox::new(s.clone()));
     nyash_rust::runtime::global_hooks::gc_alloc(s.len() as u64);
     let h = handles::to_handle_arc(arc) as i64;
-    eprintln!("[TRACE] from_i8_string -> {}", h);
     h
 }
 
@@ -263,7 +261,8 @@ pub extern "C" fn nyash_box_from_i64(val: i64) -> i64 {
     };
     let arc: std::sync::Arc<dyn NyashBox> = std::sync::Arc::new(IntegerBox::new(val));
     nyash_rust::runtime::global_hooks::gc_alloc(8);
-    handles::to_handle_arc(arc) as i64
+    let h = handles::to_handle_arc(arc) as i64;
+    h
 }
 
 // integer.get_h(handle) -> i64