🚀 Break/Continue/Try-Catch構文のサポート追加とMIRループ制御強化

## 主な変更点

### 🎯 MIRループ制御の実装（根治対応）
- src/mir/loop_builder.rs: Break/Continue対応のループコンテキスト管理
  - ループのbreak/continueターゲットブロック追跡
  - ネストループの適切な処理
- src/mir/builder.rs: Break/Continue文のMIR生成実装
- src/tokenizer.rs: Break/Continue/Tryトークン認識追加

### 📝 セルフホストパーサーの拡張
- apps/selfhost-compiler/boxes/parser_box.nyash:
  - Stage-3: break/continue構文受理（no-op実装）
  - Stage-3: try-catch-finally構文受理（構文解析のみ）
  - エラー処理構文の将来対応準備

### 📚 ドキュメント更新
- 論文K（爆速事件簿）: 45事例に更新（4件追加）
  - PyVM迂回路の混乱事件
  - Break/Continue無限ループ事件
  - EXE-first戦略の再発見
- 論文I（開発秘話）: Day 38の重要決定追加

### 🧪 テストケース追加
- apps/tests/: ループ制御とPHIのテストケース
  - nested_loop_inner_break_isolated.nyash
  - nested_loop_inner_continue_isolated.nyash
  - loop_phi_one_sided.nyash
  - shortcircuit関連テスト

## 技術的詳細
- Break/ContinueをMIRレベルで適切に処理
- 無限ループ問題（CPU 99.9%暴走）の根本解決
- 将来の例外処理機能への準備

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

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

apps/selfhost-compiler/boxes/parser_box.nyash

@@ -311,6 +311,25 @@ box ParserBox {
       me.gpos_set(j)
       return ""
     }
+    // simple assignment: IDENT '=' expr ;  → JSON v0 Local{name, expr} (Stage‑2 uses Local for updates)
+    if me.is_alpha(src.substring(j, j+1)) {
+      local idp0 = me.read_ident2(src, j)
+      local at0 = idp0.lastIndexOf("@")
+      if at0 > 0 {
+        local name0 = idp0.substring(0, at0)
+        local k0 = me.to_int(idp0.substring(at0+1, idp0.length()))
+        k0 = me.skip_ws(src, k0)
+        if src.substring(k0, k0+1) == "=" {
+          k0 = k0 + 1
+          k0 = me.skip_ws(src, k0)
+          local e0 = me.parse_expr2(src, k0)
+          k0 = me.gpos_get()
+          if k0 <= stmt_start { if k0 < src.length() { k0 = k0 + 1 } else { k0 = src.length() } }
+          me.gpos_set(k0)
+          return "{\"type\":\"Local\",\"name\":\"" + name0 + "\",\"expr\":" + e0 + "}"
+        }
+      }
+    }
     if me.starts_with_kw(src, j, "return") == 1 {
       j = j + 6
       j = me.skip_ws(src, j)
@@ -454,3 +473,70 @@ box ParserBox {
 }
 
 static box ParserStub { main(args) { return 0 } }
+    // Stage-3 acceptance (syntax only): break / continue → no-op expression
+    if me.starts_with_kw(src, j, "break") == 1 {
+      j = j + 5
+      if j <= stmt_start { if j < src.length() { j = j + 1 } else { j = src.length() } }
+      me.gpos_set(j)
+      return "{\"type\":\"Expr\",\"expr\":{\"type\":\"Int\",\"value\":0}}"
+    }
+    if me.starts_with_kw(src, j, "continue") == 1 {
+      j = j + 8
+      if j <= stmt_start { if j < src.length() { j = j + 1 } else { j = src.length() } }
+      me.gpos_set(j)
+      return "{\"type\":\"Expr\",\"expr\":{\"type\":\"Int\",\"value\":0}}"
+    }
+    // Stage-3 acceptance: throw expr → degrade to Expr(expr)
+    if me.starts_with_kw(src, j, "throw") == 1 {
+      j = j + 5
+      j = me.skip_ws(src, j)
+      local e_throw = me.parse_expr2(src, j)
+      j = me.gpos_get()
+      if j <= stmt_start { if j < src.length() { j = j + 1 } else { j = src.length() } }
+      me.gpos_set(j)
+      return "{\"type\":\"Expr\",\"expr\":" + e_throw + "}"
+    }
+    // Stage-3 acceptance: try { ... } (catch ...)* (finally { ... })? → degrade to no-op (syntax only)
+    if me.starts_with_kw(src, j, "try") == 1 {
+      j = j + 3
+      j = me.skip_ws(src, j)
+      // parse try block
+      local try_res = me.parse_block2(src, j)
+      local at_t = try_res.lastIndexOf("@")
+      j = me.to_int(try_res.substring(at_t+1, try_res.length()))
+      // zero or more catch
+      local guard_ct = 0
+      local max_ct = 100
+      local cont_ct = 1
+      loop(cont_ct == 1) {
+        if guard_ct > max_ct { cont_ct = 0 } else { guard_ct = guard_ct + 1 }
+        j = me.skip_ws(src, j)
+        if me.starts_with_kw(src, j, "catch") == 1 {
+          j = j + 5
+          j = me.skip_ws(src, j)
+          if src.substring(j, j+1) == "(" { j = j + 1 j = me.skip_ws(src, j)
+            // optional type + name
+            if me.is_alpha(src.substring(j, j+1)) { local id1 = me.read_ident2(src, j) local at1 = id1.lastIndexOf("@") j = me.to_int(id1.substring(at1+1, id1.length())) j = me.skip_ws(src, j) }
+            if me.is_alpha(src.substring(j, j+1)) { local id2 = me.read_ident2(src, j) local at2 = id2.lastIndexOf("@") j = me.to_int(id2.substring(at2+1, id2.length())) j = me.skip_ws(src, j) }
+            if src.substring(j, j+1) == ")" { j = j + 1 }
+          }
+          j = me.skip_ws(src, j)
+          // catch body
+          local c_res = me.parse_block2(src, j)
+          local atc = c_res.lastIndexOf("@")
+          j = me.to_int(c_res.substring(atc+1, c_res.length()))
+        } else { cont_ct = 0 }
+      }
+      // optional finally
+      j = me.skip_ws(src, j)
+      if me.starts_with_kw(src, j, "finally") == 1 {
+        j = j + 7
+        j = me.skip_ws(src, j)
+        local f_res = me.parse_block2(src, j)
+        local atf = f_res.lastIndexOf("@")
+        j = me.to_int(f_res.substring(atf+1, f_res.length()))
+      }
+      if j <= stmt_start { if j < src.length() { j = j + 1 } else { j = src.length() } }
+      me.gpos_set(j)
+      return "{\"type\":\"Expr\",\"expr\":{\"type\":\"Int\",\"value\":0}}"
+    }