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/compare_mir_json.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# compare_mir_json.sh — Structural diff for two MIR(JSON) files
+# Usage: tools/perf/compare_mir_json.sh <a.json> <b.json>
+# Prints sizes, sha1 (normalized), then unified diff (jq -S pretty) if available.
+
+set -euo pipefail
+
+if [[ $# -ne 2 ]]; then
+  echo "Usage: $0 <a.json> <b.json>" >&2
+  exit 2
+fi
+A="$1"; B="$2"
+if [[ ! -f "$A" || ! -f "$B" ]]; then echo "error: file not found" >&2; exit 2; fi
+
+sha1() {
+  if command -v sha1sum >/dev/null 2>&1; then sha1sum | awk '{print $1}';
+  elif command -v shasum >/dev/null 2>&1; then shasum -a 1 | awk '{print $1}';
+  else openssl sha1 | awk '{print $2}'; fi
+}
+
+size_a=$(stat -c '%s' "$A" 2>/dev/null || stat -f '%z' "$A")
+size_b=$(stat -c '%s' "$B" 2>/dev/null || stat -f '%z' "$B")
+
+norm_a=$(jq -cS . "$A" 2>/dev/null || cat "$A")
+norm_b=$(jq -cS . "$B" 2>/dev/null || cat "$B")
+
+sha_a=$(printf '%s' "$norm_a" | sha1)
+sha_b=$(printf '%s' "$norm_b" | sha1)
+
+echo "A: $A (size=$size_a, sha1=$sha_a)"
+echo "B: $B (size=$size_b, sha1=$sha_b)"
+
+if [[ "$sha_a" == "$sha_b" ]]; then
+  echo "= MIR JSON equal (normalized)"
+  exit 0
+fi
+
+echo "- Diff (normalized, jq -S)"
+tmpa=$(mktemp); tmpb=$(mktemp)
+trap 'rm -f "$tmpa" "$tmpb" || true' EXIT
+printf '%s\n' "$norm_a" | jq -S . >/dev/null 2>&1 && printf '%s\n' "$norm_a" | jq -S . >"$tmpa" || printf '%s\n' "$norm_a" >"$tmpa"
+printf '%s\n' "$norm_b" | jq -S . >/dev/null 2>&1 && printf '%s\n' "$norm_b" | jq -S . >"$tmpb" || printf '%s\n' "$norm_b" >"$tmpb"
+diff -u "$tmpa" "$tmpb" || true
+
+exit 1
+