feat(scripts): bench-ab.sh — one-command local perf A/B

scripts/bench-ab.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# A/B the gated MCP tool bench: current working tree vs a base ref.
+#
+#   scripts/bench-ab.sh              # base = HEAD (measure uncommitted work)
+#   scripts/bench-ab.sh HEAD~3       # base = any ref
+#   scripts/bench-ab.sh origin/release/0.2.5828
+#
+# Builds the base ref in a throwaway worktree under $HOME (tests and bench
+# misbehave under /tmp roots — see root_policy), runs `zig build bench --
+# --json` for both sides on this machine back-to-back, and prints the same
+# regression table CI posts on PRs (scripts/compare-bench.py, 10% + 50µs
+# thresholds). Only same-machine, same-run deltas are meaningful — never
+# compare against a JSON from another day or box.
+set -euo pipefail
+
+REPO_ROOT="$(git rev-parse --show-toplevel)"
+BASE_REF="${1:-HEAD}"
+BASE_SHA="$(git -C "$REPO_ROOT" rev-parse --short "$BASE_REF")"
+WT="$HOME/.cache/codedb-bench-ab-$$"
+OUT="$(mktemp -d "${TMPDIR:-/tmp}/codedb-bench-ab.XXXXXX")"
+
+cleanup() {
+  git -C "$REPO_ROOT" worktree remove --force "$WT" >/dev/null 2>&1 || true
+  rm -rf "$OUT"
+}
+trap cleanup EXIT
+
+echo "base:  $BASE_REF ($BASE_SHA) in throwaway worktree"
+echo "head:  working tree at $(git -C "$REPO_ROOT" rev-parse --short HEAD)$(git -C "$REPO_ROOT" diff --quiet || echo ' + uncommitted changes')"
+
+git -C "$REPO_ROOT" worktree add --detach "$WT" "$BASE_REF" >/dev/null
+(cd "$WT" && python3 scripts/run-bench-json.py "$OUT/base.json" >/dev/null)
+(cd "$REPO_ROOT" && python3 scripts/run-bench-json.py "$OUT/head.json" >/dev/null)
+
+python3 "$REPO_ROOT/scripts/compare-bench.py" "$OUT/base.json" "$OUT/head.json" \
+  --threshold-pct 10 --min-abs-ns 50000