Phase 21.7 — Normalization & Unification (Methodize Static Boxes)

Goal
- Unify user-defined function calls onto a single, consistent representation.
- Move from ad-hoc Global("Box.method") calls toward Method calls with an explicit (singleton) receiver when appropriate.
- Keep defaults stable; introduce dev toggles and canaries; then promote to default after green.

Scope
- Parser/Stage‑B: keep emitting Program(JSON v0). No schema changes required for MVP.
- MirBuilder: add methodization (dev toggle) that:
  - Emits method functions as-is (defs) and/or provides a mapping to Method calls
  - Rewrites Global("Box.method") to Method {receiver=static singleton, box_name, method}
  - Preserves arity and naming as Box.method/N
- VM: handle Method calls uniformly (receiver resolved via ensure_static_box_instance for static boxes).
- LLVM: rely on mir_call(Method) lowering (already supported) and provide the static receiver where needed.

Toggles
- HAKO_MIR_BUILDER_METHODIZE=1 — enable methodization (rewrite Global→Method with static singleton receiver)
- HAKO_STAGEB_FUNC_SCAN=1 — Stage‑B defs scan (already available)
- HAKO_MIR_BUILDER_FUNCS=1 — defs→MIR 関数化（既存）
- HAKO_MIR_BUILDER_CALL_RESOLVE=1 — Global 名解決（既存）

Design Rules
- Naming: canonical "Box.method/N". Arity N must equal params.len (or callsite args len if defs unknown).
- Receiver: for static boxes, provide implicit singleton receiver. For instance boxes, preserve existing Method path.
- Effects: preserve EffectMask semantics; do not broaden side effects.
- Fail‑Fast: when rewrite is ambiguous (multiple candidates), keep original form and WARN under dev toggle; never change defaults silently.

Acceptance
- Canaries:
  - call (global style) passes when methodization ON (rc unchanged)
  - Existing return/binop/loop/call (global) canaries remain green when OFF
  - A dedicated methodization canary asserts presence of callee.type=="Method" in MIR(JSON v1) or correct v0 lowering

Rollout Plan
1) Ship behind HAKO_MIR_BUILDER_METHODIZE=1 with canaries
2) Validate on representative apps (selfhost‑first path)
3) Promote to default ON only after green for a cycle; keep rollback instructions

Rollback
- Disable HAKO_MIR_BUILDER_METHODIZE. Revert to Global("Box.method") resolution path (current 21.6 behavior).

Current notes (Phase 25.x bring-up)
- NamingBox / static 名 SSOT
  - `src/mir/naming.rs` に NamingBox を実装済み。`Main.main` / `main._nop` などの揺れを `"Main.main/0"` 形式に正規化する経路は Rust VM/LLVM/JSON bridge から既に利用中。
  - VM 側は `normalize_static_global_name` を通して static box 名を一元化するよう更新済み。

- 既知のギャップ
  - static box 内のローカル呼び出し（例: `Main.main` → `me._nop()`）が Global 呼び出しのまま落ちるケースを確認済み。NamingBox で `main`→`Main` 正規化は済んでいるが、「Method 化＋receiver 付与」が未完了。
  - MeCallPolicy / method_call_handlers 周りで、static box method に対しても一律で receiver（`me`）を先頭に追加してしまうパスがあり、arity 不一致や miss-call の温床になり得る。

- このフェーズでやること（具体タスク）
  1. `src/mir/builder/method_call_handlers.rs` / MeCallPolicy で「static box method かどうか」を判定し、static のときは receiver を追加しない（args はそのまま Global 互換で流す）ガードを入れる。
  2. methodization ロジック（HAKO_MIR_BUILDER_METHODIZE=1 有効時）で、static box 呼びを `ensure_static_box_instance(Box)` 経由の Method 呼び出しに寄せる:
     - Global("Main._nop/0") → Method{ receiver = ensure_static_box_instance("Main"), box="Main", method="_nop", arity=0 }。
  3. 最小テストを追加:
     - `.hako` 側 minimal: `static box Main { static method _nop() { return 0 } }` を呼ぶケース。
     - Rust 側: `mir_stage1_static_main_nop_resolves_and_execs`（仮）で、MIR verify + VM 実行が methodization ON/OFF の両方で安定することを確認。
  4. docs: 本ファイルに「static box 正規化の完了条件」と「NamingBox / ensure_static_box_instance / MeCallPolicy の責務分担」を 1 ページにまとめ、Phase 21.7 の完了ラインを明文化する。