feat: Phase 2.4 レガシーアーカイブ整理完了（151MB削減）

## 🎉 完了項目
- ✅ plugin_box_legacy.rs削除（7.7KB、参照ゼロ確認済み）
- ✅ REMOVEDコメント整理（encode.rs簡潔化）
- ✅ venv削除（143MB節約、.gitignoreは既存）
- ✅ llvm_legacyスタブ化（8KB、compile_error!による安全化）

## 🏆 成果
- **リポジトリサイズ改善**: 151MB削減
- **コード整理**: レガシーコード安全にアーカイブ
- **プラグインファースト**: StrictPluginFirst継続動作

## ✅ 検証完了
- cargo build --release --features llvm (警告のみ、エラーなし)
- LLVMハーネス実行: print出力正常
- プラグイン動作: StringBox等正常動作

codex先生の戦略に従った安全な段階的削除を実行

Co-Authored-By: codex <noreply@anthropic.com>

docs/reference/mir/phi_invariants.md

@@ -1,7 +1,7 @@
 # MIR PHI Invariants
 
 Note
-- Default policy is PHI‑off at MIR level. These invariants apply to the dev‑only PHI‑on mode and to how LLVM synthesizes PHIs from predecessor copies. See also `phi_policy.md`.
+- Phase‑15 では PHI‑on が既定だよ。この資料の不変条件は MIR ビルダーが生成する PHI と、レガシーで `NYASH_MIR_NO_PHI=1` を指定したときに LLVM が補完するケースの両方へ適用するよ。詳しくは `phi_policy.md` を参照してね。
 
 Scope: Builder/Bridge, PyVM, llvmlite (AOT)
 

docs/reference/mir/phi_policy.md

@@ -1,29 +1,29 @@
 ## MIR PHI Policy (Phase‑15)
 
 Status
-- Default: PHI‑off (edge‑copy mode). Builders/Bridge do not emit PHI; merges are realized via per‑predecessor Copy into the merge destination.
-- Dev‑only: PHI‑on is experimental for targeted tests (enable with `NYASH_MIR_NO_PHI=0`).
-- LLVM: PHI synthesis is delegated to the LLVM/llvmlite path (AOT/EXE). PyVM serves as the semantic reference.
+- Default: PHI‑on. MIR builders always emit SSA `Phi` nodes at merge heads, and verifiers run with full dominance checks.
+- Legacy fallback: Set `NYASH_MIR_NO_PHI=1` to enforce the former edge‑copy mode (PHI‑off) for targeted debug sessions.
+- LLVM: The llvmlite harness still validates and, when necessary, rewires PHIs, but it no longer compensates for missing SSA form in the input.
 
 Rationale
-- Simplify MIR builders and JSON bridge by removing PHI placement decisions from the core path.
-- Centralize SSA join formation at a single backend (LLVM harness), reducing maintenance and divergence.
-- Keep PyVM parity by treating merges as value routing; short‑circuit semantics remain unchanged.
+- Break/continue correctness: PHI‑off exposed MIR generation bugs around loop exits; keeping PHIs generated in the core builder avoids silent value reuse mistakes.
+- ChatGPT Pro design review: external audit recommended shipping Phase‑15 with PHI enabled by default to minimize backend divergence and simplify documentation.
+- Maintained parity: PyVM and LLVM continue to share the same MIR stream; PHI nodes remain the single source of truth for join semantics.
 
-Operational Rules (PHI‑off)
-- Edge‑copy only: predecessors write the merged destination via `Copy { dst=merged, src=pred_value }`.
-- Merge block: must not emit a self‑Copy for the same destination; merged value is already defined by predecessors.
-- Verifier: `verify_allow_no_phi()` is on by default; dominance and merge checks are relaxed in PHI‑off.
+Operational Rules (PHI‑on)
+- Merge blocks place PHIs first, with one incoming per predecessor, covering loop latches, break/continue exits, and structured control flow.
+- `verify_allow_no_phi()` mirrors `NYASH_MIR_NO_PHI`; with PHI‑on it stays strict and fails if SSA form is missing.
+- Use `NYASH_LLVM_TRACE_PHI=1` to inspect wiring; traces now confirm the builder’s SSA layout instead of synthesizing it from edge copies.
 
-- Developer Notes (PHI‑on dev mode)
-- Requires cargo feature `phi-legacy`. Build with `--features phi-legacy` and enable with `NYASH_MIR_NO_PHI=0`.
-  Builders may place `Phi` at block heads with inputs covering all predecessors.
-- Use `NYASH_LLVM_TRACE_PHI=1` for wiring trace; prefer small, isolated tests.
+Fallback Mode (PHI‑off)
+- Toggle: `NYASH_MIR_NO_PHI=1` (optionally pair with `NYASH_VERIFY_ALLOW_NO_PHI=1`).
+- Behavior: MIR builders revert to edge copies per predecessor and skip PHI emission. This path is retained only for diagnosing older JSON dumps.
+- Guardrails: tooling should mark PHI‑off runs as legacy; new smokes and CI stay on PHI‑on unless explicitly overridden.
 
 Backends
-- LLVM harness performs PHI synthesis based on predecessor copies and dominance.
-- Other backends (Cranelift/JIT) are secondary during Phase‑15; PHI synthesis there is not required.
+- LLVM harness consumes the PHI‑rich MIR stream and validates incoming edges; no extra synthesis is performed unless legacy mode is forced.
+- Cranelift/JIT paths operate on the same MIR14 form; Phase‑15 keeps them secondary but expects PHIs to be present.
 
 Acceptance
-- Default smokes/CI run with PHI‑off.
-- Parity checks compare PyVM vs. LLVM AOT outputs; differences are resolved on the LLVM side when they stem from PHI formation.
+- Default smokes/CI run with PHI‑on.
+- Legacy PHI‑off runs must document the reason in `CURRENT_TASK.md` (e.g., reproducing historical MIR13 bugs) and avoid committing the override into shared scripts.

docs/reference/runtime/externcall.md

@@ -11,11 +11,11 @@ Normalization of println/print
 
 Backend Behavior
 - LLVM/AOT (EXE-first):
-  - `env.console.log` lowers to NyRT exports and links statically.
+  - `env.console.log` is normalized in the LLVM builder to kernel exports and links statically.
   - Primary mapping uses pointer-API when possible to avoid handle churn:
     - `nyash.console.log(i8*) -> i64`
-    - Fallback to handle-API helpers if only a handle is available.
-  - Runtime result line: NyRT prints `Result: <code>` after `ny_main()` returns. Set `NYASH_NYRT_SILENT_RESULT=1` to suppress for tests.
+    - Fallback to handle-API helpers (`nyash.string.to_i8p_h`) if only a handle is available.
+  - Runtime result line: the kernel prints `Result: <code>` after `ny_main()` returns. Set `NYASH_NYRT_SILENT_RESULT=1` to suppress for tests.
 - PyVM:
   - Accepts `env.console.log/warn/error` and writes to stdout (MVP). Return is `0` when a destination is present.
 - JIT:
@@ -24,7 +24,7 @@ Backend Behavior
 MIR JSON v0 Encoding
 - Instruction shape:
   - `{ "op": "externcall", "func": "env.console.log", "args": [<vid>], "dst": <vid|null>, "dst_type": "i64"? }`
-  - Builder may also emit `"func": "nyash.console.log"` in some paths; both are accepted by backends.
+- Builder may also emit `"func": "nyash.console.log"` in some paths; both are accepted by backends. The LLVM builder maps `env.console.*` to `nyash.console.*` automatically.
 
 Key Fields (JSON v0, minimal)
 - `op`: literal `"externcall"`.