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).