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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tools/perf/bench_ny_mir_builder.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+# bench_ny_mir_builder.sh — Quick micro-bench for MIR(JSON) → {obj|exe}
+# Usage: tools/perf/bench_ny_mir_builder.sh <mir.json> [rounds]
+# Notes:
+#  - Uses crate backend (ny-llvmc). Keeps defaults conservative (O0).
+#  - Prints simple CSV: kind,round,ms
+
+set -euo pipefail
+
+if [[ $# -lt 1 ]]; then
+  echo "Usage: $0 <mir.json> [rounds]" >&2
+  exit 2
+fi
+IN="$1"; ROUNDS="${2:-3}"
+
+BIN_BUILDER="tools/ny_mir_builder.sh"
+if [[ ! -x "$BIN_BUILDER" ]]; then echo "error: $BIN_BUILDER not found/executable" >&2; exit 2; fi
+
+measure() {
+  local kind="$1"; shift
+  local out_path="$PWD/target/aot_objects/__bench_${kind}_$$"
+  [[ "$kind" == "exe" ]] && out_path+=".out" || out_path+=".o"
+  local start end ms
+  start=$(date +%s%3N)
+  NYASH_LLVM_BACKEND=crate "$BIN_BUILDER" --in "$IN" --emit "$kind" -o "$out_path" --quiet || return 1
+  end=$(date +%s%3N)
+  ms=$((end - start))
+  rm -f "$out_path" 2>/dev/null || true
+  echo "$kind,$ms"
+}
+
+echo "kind,round,ms"
+for k in obj exe; do
+  for ((i=1; i<=ROUNDS; i++)); do
+    line=$(measure "$k" || echo "$k,ERROR")
+    echo "$k,$i,${line#*,}"
+  done
+done
+