perf(utils): reduce highway hash key setup (#5434)

* fix: keep hotpath gate csv paths clean

Redirect the enhanced bench driver's verbose output away from command substitution so run_hotpath_warp_ab.sh records only the candidate cell path before passing baseline_compare.csv files to the gate.

Co-Authored-By: heihutu <heihutu@gmail.com>

* perf(utils): reduce highway hash key setup

Cache the parsed HighwayHash keys as compile-time constants and add a criterion benchmark for the PUT hash hotpath.

Co-Authored-By: heihutu <heihutu@gmail.com>

---------

Co-authored-by: heihutu <heihutu@gmail.com>

scripts/run_hotpath_warp_ab.sh

@@ -337,7 +337,7 @@ measure() {
   )
   [[ -n "$COOLDOWN_SECS" ]] && args+=(--cooldown-secs "$COOLDOWN_SECS")
   [[ -n "$baseline_csv" ]] && args+=(--baseline-csv "$baseline_csv")
-  run "$ENHANCED_BENCH" "${args[@]}"
+  run "$ENHANCED_BENCH" "${args[@]}" >&2
   echo "$cell"
 }