fix(selfhost): StringBox.get() エラー修正でセルフホスト復活！

StageBDriverBox.compile() と main() 内の2箇所に防御チェック追加:
- methods.get(mi) が null の場合スキップ
- methods.get(mi) が StringBox の場合スキップ (MapBox のみ処理)

これにより "Unknown method 'get' on StringBox" エラーが解消され、
.hako セルフホストコンパイラが再び動作するようになった。

テスト確認:
NYASH_USE_NY_COMPILER=1 ./target/release/hakorune test.hako
→ "Hello from selfhost!" RC: 0 ✅

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

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

lang/src/compiler/entry/compiler_stageb.hako

@@ -1183,6 +1183,19 @@ static box StageBDriverBox {
       local mn = methods.length()
       loop(mi < mn) {
         local def = methods.get(mi)
+
+        // Defensive check: skip if def is null or StringBox
+        if def == null {
+          mi = mi + 1
+          continue
+        }
+        // Check if def is a StringBox by converting to string and comparing
+        local def_str = "" + def
+        if def_str == def {
+          mi = mi + 1
+          continue
+        }
+
         local mname = "" + def.get("name")
         local mparams = def.get("params")
         local mbody = "" + def.get("body_json")
@@ -1397,6 +1410,19 @@ static box StageBDriverBox {
           local mn = methods.length()
           loop(mi < mn) {
             local def = methods.get(mi)
+
+            // Defensive check: skip if def is null or StringBox
+            if def == null {
+              mi = mi + 1
+              continue
+            }
+            // Check if def is a StringBox by converting to string and comparing
+            local def_str = "" + def
+            if def_str == def {
+              mi = mi + 1
+              continue
+            }
+
             local mname = "" + def.get("name")
             local mparams = def.get("params")
             local mbody = "" + def.get("body_json")