🚀 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>

apps/selfhost-compiler/emitter/json_v0.nyash

@@ -0,0 +1,8 @@
+// JSON v0 emitter (MVP placeholder)
+static box JsonV0Emitter {
+  // Emit a minimal Program{return 0}
+  program_return0() {
+    return "{\"version\":0,\"kind\":\"Program\",\"body\":[{\"type\":\"Return\",\"expr\":{\"type\":\"Int\",\"value\":0}}]}"
+  }
+}
+