🚀 Start Phase 15.3: Nyash compiler MVP implementation

Major milestone:
- Set up apps/selfhost-compiler/ directory structure
- Implement basic Nyash compiler in Nyash (CompilerBox)
- Stage-1: Basic arithmetic parser (int/string/+/-/*/括弧/return)
- JSON v0 output compatible with --ny-parser-pipe
- Runner integration with NYASH_USE_NY_COMPILER=1 flag
- Comprehensive smoke tests for PHI/Bridge/Stage-2

Technical updates:
- Updated CLAUDE.md with Phase 15.3 status and MIR14 details
- Statement separation policy: newline-based with minimal ASI
- Fixed runaway ny-parser-pipe processes (CPU 94.9%)
- Clarified MIR14 as canonical instruction set (not 13/18)
- LoopForm strategy: PHI auto-generation during reverse lowering

Collaborative development:
- ChatGPT5 implementing compiler skeleton
- Codex provided LoopForm PHI generation guidance
- Claude maintaining documentation and coordination

🎉 セルフホスティングの歴史的一歩！自分自身をコンパイルする日が近いにゃ！

Co-Authored-By: ChatGPT <noreply@openai.com>

src/runner/json_v0_bridge.rs

@@ -242,10 +242,23 @@ fn lower_expr_with_vars(
     match e {
         ExprV0::Var { name } => {
             if let Some(&vid) = vars.get(name) {
-                Ok((vid, cur_bb))
-            } else {
-                Err(format!("undefined variable: {}", name))
+                return Ok((vid, cur_bb));
             }
+            if name == "me" {
+                // Optional gate: allow a dummy 'me' instance for Stage-2 JSON smoke
+                if std::env::var("NYASH_BRIDGE_ME_DUMMY").ok().as_deref() == Some("1") {
+                    let class = std::env::var("NYASH_BRIDGE_ME_CLASS").unwrap_or_else(|_| "Main".to_string());
+                    let dst = f.next_value_id();
+                    if let Some(bb) = f.get_block_mut(cur_bb) {
+                        bb.add_instruction(MirInstruction::NewBox { dst, box_type: class, args: vec![] });
+                    }
+                    vars.insert("me".to_string(), dst);
+                    return Ok((dst, cur_bb));
+                } else {
+                    return Err("undefined 'me' outside box context (set NYASH_BRIDGE_ME_DUMMY=1 to inject placeholder)".into());
+                }
+            }
+            Err(format!("undefined variable: {}", name))
         }
         ExprV0::Call { name, args } => {
             // Lower args