Fix CSV path joining and relative exclusions; add scan benchmark script

scripts/bench-scan.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# Records scan benchmarks as Markdown table rows for PERFORMANCE.md.
+#
+#   scripts/bench-scan.sh PATH [workers...]      (default workers: 1 2 4 8 16)
+#
+# Linux only (uses /usr/bin/time, df, lsblk). Runs each configuration three
+# times against a warm cache; to measure a cold cache, run as root with
+# DROP_CACHES=1, which drops the page cache before every run.
+set -euo pipefail
+path=${1:?usage: bench-scan.sh PATH [workers...]}
+shift || true
+workers=${*:-1 2 4 8 16}
+bin=${MAPSIZE:-bin/mapsize}
+[[ -x $bin ]] || make build >/dev/null
+
+fs=$(df -T "$path" | awk 'NR==2 {print $2}')
+dev=$(df "$path" | awk 'NR==2 {print $1}')
+medium=$(lsblk -no ROTA "$dev" 2>/dev/null | head -1 | sed 's/1/rotational/;s/0/solid-state/')
+counts=$("$bin" --no-ui "$path" 2>/dev/null | awk '/^Files/ {f=$2} /^Directories/ {d=$2} END {print f" files, "d" dirs"}')
+
+echo "<!-- $(date -u +%FT%TZ) $(uname -srm), $(nproc) CPUs -->"
+echo "Path: \`$path\` · $fs on ${medium:-unknown medium} · $counts"
+echo
+echo "| workers | cache | wall (s) | peak RSS (MB) |"
+echo "|---:|---|---:|---:|"
+for w in $workers; do
+  for i in 1 2 3; do
+    cache=warm
+    if [[ ${DROP_CACHES:-} == 1 ]]; then
+      sync; echo 3 > /proc/sys/vm/drop_caches; cache=cold
+    fi
+    out=$( { /usr/bin/time -f "%e %M" "$bin" --no-ui --workers "$w" "$path" >/dev/null; } 2>&1 | tail -1)
+    read -r secs kb <<<"$out"
+    echo "| $w | $cache | $secs | $((kb / 1024)) |"
+  done
+done