Phase 21.7 normalization: optimization pre-work + bench harness expansion

- Add opt-in optimizations (defaults OFF)
  - Ret purity verifier: NYASH_VERIFY_RET_PURITY=1
  - strlen FAST enhancement for const handles
  - FAST_INT gate for same-BB SSA optimization
  - length cache for string literals in llvmlite
- Expand bench harness (tools/perf/microbench.sh)
  - Add branch/call/stringchain/arraymap/chip8/kilo cases
  - Auto-calculate ratio vs C reference
  - Document in benchmarks/README.md
- Compiler health improvements
  - Unify PHI insertion to insert_phi_at_head()
  - Add NYASH_LLVM_SKIP_BUILD=1 for build reuse
- Runtime & safety enhancements
  - Clarify Rust/Hako ownership boundaries
  - Strengthen receiver localization (LocalSSA/pin/after-PHIs)
  - Stop excessive PluginInvoke→BoxCall rewrites
- Update CURRENT_TASK.md, docs, and canaries

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

docs/development/cleanup/legacy-byname-removal.md

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+Legacy By‑Name Call Removal Plan
+
+Context
+- Unified call system is default. VM routes by `Callee` kind (Global/Method/Extern/…)
+- Legacy by‑name resolution (callee=None → string name lookup) is gated by env and OFF by default.
+- Goal: remove legacy code path once parity is proven at integration/full levels.
+
+Guards / Current Behavior
+- 旧 by‑name 経路は削除済み。`NYASH_VM_ALLOW_LEGACY` は廃止。
+- 既定: `callee=None` 生成時は Fail‑Fast（Builder で Callee を必ず付与）。
+
+Scope of Work
+1) Identify callers that could still rely on legacy resolution
+   - Builder emissions that may omit `callee` (should be none after Phase 1 refactor)
+   - Any VM entry points or helpers constructing by‑name calls dynamically
+   - Older tests or tools expecting by‑name lookups
+
+2) Ensure unified path covers them
+   - For Global user functions: attach `Callee::Global("Module.func")`
+   - For dynamic/indirect calls: mark `Callee::Value` and keep VM restrictions explicit
+   - For extern‑like names: delegate via handlers/calls/externs.rs (SSOT)
+   - Normalize arity (`foo/1` → `foo`) via `utils::normalize_arity_suffix`
+
+3) Strengthen tests
+   - Smokes/quick: unified path（by‑name 不可）を既に強制
+   - Integration/full: ユーザー関数呼び出し/extern 名正規化のカナリアを追加
+   - 旧ENV依存がないことを確認（`NYASH_VM_ALLOW_LEGACY` は削除済）
+
+4) Remove legacy code path
+   - Delete `src/backend/mir_interpreter/handlers/calls/legacy.rs` and its resolver module
+   - Drop env guard checks referring to legacy path (keep error message clarity)
+   - Update READMEs to remove temporary notes
+
+Readiness Criteria (before removal)
+- quick/integration/full all green with legacy OFF
+- No `callee=None` emission in builder (verified by grepping MIR JSON / code review)
+- Extern SSOT accepts arity‑suffixed names; Global extern‑like names delegate properly
+
+Rollback Plan
+- Keep a small revertable commit boundary for legacy deletion
+- If issues appear, reintroduce legacy.rs with the same path under a dev‑guard until fixed
+
+Cross‑References
+- handlers/calls/README.md (SSOT and boundaries)
+- docs/ENV_VARS.md (env guards and policies)