🚀 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/language/statements.md

@@ -0,0 +1,70 @@
+# Statement Separation and Semicolons
+
+Status: Adopted for Phase 15.3+; parser implementation is staged.
+
+Policy
+- Newline as primary statement separator.
+- Semicolons are optional and only needed when multiple statements appear on one physical line.
+- Minimal ASI (auto semicolon insertion) rules to avoid surprises.
+
+Rules (minimal and predictable)
+- Newline ends a statement when:
+  - Parenthesis/brace/bracket depth is 0, and
+  - The line does not end with a continuation token (`+ - * / . ,` etc.).
+- Newline does NOT end a statement when:
+  - Inside any open grouping `(...)`, `[...]`, `{...}`; or
+  - The previous token is a continuation token.
+- `return/break/continue` end the statement at newline unless the value is on the same line or grouped via parentheses.
+- `if/else` (and similar paired constructs): do not insert a semicolon between a block and a following `else`.
+- One‑line multi‑statements are allowed with semicolons: `x = 1; y = 2; print(y)`.
+- Method chains can break across lines after a dot: `obj\n  .method()` (newline treated as whitespace).
+
+Style guidance
+- Prefer newline separation (no semicolons) for readability.
+- Use semicolons only when placing multiple statements on a single line.
+
+Examples
+```nyash
+// Preferred (no semicolons)
+local x = 5
+x = x + 1
+print(x)
+
+// One line with multiple statements (use semicolons)
+local x = 5; x = x + 1; print(x)
+
+// Line continuation by operator
+local n = 1 +
+          2 +
+          3
+
+// Grouping across lines
+return (
+  1 + 2 + 3
+)
+
+// if / else on separate lines without inserting a semicolon
+if cond {
+  x = x - 1
+}
+else {
+  print(x)
+}
+
+// Dot chain across lines
+local v = obj
+  .methodA()
+  .methodB(42)
+```
+
+Implementation notes (parser)
+- Tokenizer keeps track of grouping depth.
+- At newline, attempt ASI only when depth==0 and previous token is not a continuation.
+- Error messages should suggest adding a continuation token or grouping when a newline unexpectedly ends a statement.
+
+Parser dev notes (Stage‑1/2)
+- return + newline: treat bare `return` as statement end. To return an expression on the next line, require grouping with parentheses.
+- if/else: never insert a semicolon between a closed block and `else` (ASI禁止箇所)。
+- Dot chains: treat `.` followed by newline as whitespace (line continuation)。
+- One‑line multi‑statements: accept `;` as statement separator, but formatter should prefer newlines.
+- Unary minus: disambiguate from binary minus; implement after Stage‑1（当面は括弧で回避）。