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>

src/llvm_py/builders/function_lower.py

@@ -62,6 +62,14 @@ def lower_function(builder, func_data: Dict[str, Any]):
             builder.resolver.string_literals.clear()
         if hasattr(builder.resolver, 'string_ptrs'):
             builder.resolver.string_ptrs.clear()
+        if hasattr(builder.resolver, 'length_cache'):
+            builder.resolver.length_cache.clear()
+        # Also clear newbox→string-arg hints per function to avoid leakage
+        try:
+            if hasattr(builder.resolver, 'newbox_string_args') and isinstance(builder.resolver.newbox_string_args, dict):
+                builder.resolver.newbox_string_args.clear()
+        except Exception:
+            pass
     except Exception:
         pass