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/ny_mir_builder.sh

@@ -186,7 +186,8 @@ case "$EMIT" in
         fi
         # Produce exe directly via ny-llvmc (lets ny-llvmc link)
         LIBS="${HAKO_AOT_LDFLAGS:-}"
-        if ! "$BIN_NYLLVMC" --in "$IN_FILE" --emit exe --nyrt target/release --libs "$LIBS" --out "$OUT" >/dev/null 2>&1; then
+        # Run and surface linker diagnostics on failure
+        if ! "$BIN_NYLLVMC" --in "$IN_FILE" --emit exe --nyrt target/release --libs "$LIBS" --out "$OUT"; then
           echo "error: ny-llvmc failed to link exe" >&2; exit 4
         fi
         ;;