🚀 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>

docs/guides/language-guide.md

@@ -52,3 +52,7 @@ When you need the implementation details
 - Tokenizer: src/tokenizer.rs
 - Parser: src/parser/expressions.rs, src/parser/statements.rs
 - Lowering to MIR: src/mir/builder/**
+Statement Separation (Semicolons)
+- Newline separates statements by default; semicolons are optional.
+- Use semicolons only when placing multiple statements on one line.
+- Minimal ASI rules: newline does not end a statement when the line ends with an operator/dot/comma, or while inside grouping.