feat(phase-9.77): Implement BoxCall instructions and fix wasmtime version

Phase 9.77 WASM Emergency Recovery Progress:
- ✅ Task 1.1: Implement BoxCall instruction for toString(), print(), equals(), clone(), log()
- ✅ Task 1.2: Update wasmtime 18.0 → 35.0.0 and add runtime imports
- 🔄 Task 1.3: Working on UTF-8 encoding error fix

Changes:
- Add generate_box_call() method in codegen.rs with 5 helper methods
- Update wasmtime dependency to 35.0.0 for AOT compatibility
- Add BoxCall runtime imports (box_to_string, box_print, box_equals, box_clone)
- Implement wat_to_wasm() with UTF-8 validation and debug output
- Update CURRENT_TASK.md with Copilot handoff notes

Current issue: 'Generated WASM is not valid UTF-8' error source unknown
Next: Copilot to investigate error origin and complete Task 1.3

🤖 Generated with Claude Code

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

local_tests/debug_wasm_generation.nyash

@@ -0,0 +1,8 @@
+// 🧪 最も単純なWASM生成テスト
+// 変数操作も避けて、純粋な計算のみ
+
+static box Main {
+    main() {
+        return "成功"
+    }
+}

local_tests/test_mir_wasm_minimal.nyash

@@ -0,0 +1,17 @@
+// 🧪 MIR → WASM変換テスト（最小版）
+// メソッド呼び出しを避けて基本演算のみテスト
+
+static box Main {
+    init { result }
+    
+    main() {
+        // 基本演算のみ（メソッド呼び出しなし）
+        local a = 10
+        local b = 20
+        local sum = a + b
+        local product = sum * 3
+        
+        me.result = product
+        return "計算完了"
+    }
+}

local_tests/test_mir_wasm_simple.nyash

@@ -0,0 +1,28 @@
+// 🧪 MIR → WASM変換テスト用シンプルプログラム
+// 基本的な演算とBox操作をテスト
+
+static box Main {
+    init { console, result }
+    
+    main() {
+        me.console = new ConsoleBox()
+        me.console.log("🚀 MIR → WASM変換テスト開始")
+        
+        // 基本演算テスト
+        local a = 10
+        local b = 20
+        local sum = a + b
+        
+        me.console.log("計算結果: " + sum.toString())
+        
+        // StringBox操作テスト
+        local greeting = "Hello"
+        local name = "WASM"
+        local message = greeting + " " + name + "!"
+        
+        me.console.log("メッセージ: " + message)
+        
+        me.result = sum
+        return "MIR → WASM変換テスト完了"
+    }
+}

local_tests/test_simple_wasm.nyash

@@ -0,0 +1 @@
+local result = 42