🚀 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/reference/ir/json_v0.md

@@ -0,0 +1,81 @@
+# Ny JSON IR v0 — Minimal Spec (Stage‑2)
+
+Status: experimental but stable for Phase‑15 Stage‑2. Input to `--ny-parser-pipe`.
+
+Version and root
+- `version`: 0
+- `kind`: "Program"
+- `body`: array of statements
+
+Statements (`StmtV0`)
+- `Return { expr }`
+- `Extern { iface, method, args[] }` (optional; passes through to `ExternCall`)
+- `Expr { expr }` (expression statement; side effects only)
+- `Local { name, expr }` (Stage‑2)
+- `If { cond, then: Stmt[], else?: Stmt[] }` (Stage‑2)
+- `Loop { cond, body: Stmt[] }` (Stage‑2; while(cond) body)
+
+Expressions (`ExprV0`)
+- `Int { value }` where `value` is JSON number or digit string
+- `Str { value: string }`
+- `Bool { value: bool }`
+- `Binary { op: "+"|"-"|"*"|"/", lhs, rhs }`
+- `Compare { op: "=="|"!="|"<"|"<="|">"|">=", lhs, rhs }`
+- `Logical { op: "&&"|"||", lhs, rhs }` (short‑circuit)
+- `Call { name: string, args[] }` (function by name)
+- `Method { recv: Expr, method: string, args[] }` (box method)
+- `New { class: string, args[] }` (construct Box)
+- `Var { name: string }`
+
+CFG conventions (lowered by the bridge)
+- If: create `then_bb`, `else_bb`, `merge_bb`. Both branches jump to merge if unterminated.
+- Loop: `preheader -> cond_bb -> (body_bb or exit_bb)`, body jumps back to cond.
+- Short‑circuit Logical: create `rhs_bb`, `fall_bb`, `merge_bb` with constants on fall path.
+- All blocks end with a terminator (branch/jump/return).
+
+PHI merging (current behavior)
+- If: locals updated in `then`/`else` merge at `merge_bb` via `phi`.
+  - Else欠落時は else 側に分岐前(base)を採用。
+  - 片側にしか存在しない新規変数はスコープ外として外へ未伝播。
+- Loop: `cond_bb` にヘッダ PHI を先置き（preheader/base と latch/body end を合流）。
+- 目的: Stage‑2 を早期に安定化させるための橋渡し。将来（Core‑14）は LoopForm からの逆LoweringでPHI自動化予定。
+
+Type meta (emitter/LLVM harness cooperation)
+- `+` with any string operand → string concat path（handle固定）。
+- `==/!=` with both strings → string compare path。
+
+Special notes
+- `Var("me")`: Bridge 既定では未定義エラー。デバッグ用に `NYASH_BRIDGE_ME_DUMMY=1` でダミー `NewBox{class}` を注入可（`NYASH_BRIDGE_ME_CLASS` 省略時は `Main`）。
+- `--ny-parser-pipe` は stdin の JSON v0 を受け取り、MIR→MIR‑Interp 経由で実行する。
+
+CLI/Env cheatsheet
+- Pipe: `echo '{...}' | target/release/nyash --ny-parser-pipe`
+- File: `target/release/nyash --json-file sample.json`
+- Verbose MIR dump: `NYASH_CLI_VERBOSE=1`
+- me dummy: `NYASH_BRIDGE_ME_DUMMY=1 NYASH_BRIDGE_ME_CLASS=ConsoleBox`
+
+Examples
+
+Arithmetic
+```json
+{"version":0,"kind":"Program","body":[
+  {"type":"Return","expr":{
+    "type":"Binary","op":"+",
+    "lhs":{"type":"Int","value":1},
+    "rhs":{"type":"Binary","op":"*","lhs":{"type":"Int","value":2},"rhs":{"type":"Int","value":3}}
+  }}
+]}
+```
+
+If with local + PHI merge
+```json
+{"version":0,"kind":"Program","body":[
+  {"type":"Local","name":"x","expr":{"type":"Int","value":1}},
+  {"type":"If","cond":{"type":"Compare","op":"<","lhs":{"type":"Int","value":1},"rhs":{"type":"Int","value":2}},
+    "then":[{"type":"Local","name":"x","expr":{"type":"Int","value":10}}],
+    "else":[{"type":"Local","name":"x","expr":{"type":"Int","value":20}}]
+  },
+  {"type":"Return","expr":{"type":"Var","name":"x"}}
+]}
+```
+