test(bench): SponzaBench harness + #40→HEAD perf measurement (#155)

Headless benchmark around the native Sponza RT scene: times setup and a
measured Render() loop over the full multi-mesh atrium, prints BENCH
metrics, and exits. Includes run-bench.sh and a README documenting the
methodology and the measured net gain from #40 to current master.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/SponzaBench/run-bench.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Repeated-measurement harness for SponzaBench. Runs the binary in $BINDIR
+# REPS times for a given mesh cap and reports the median (and min/max) of
+# each BENCH metric. Validation is force-disabled via the loader (it adds
+# huge, version-dependent overhead) and present mode is uncapped so frame
+# time reflects real work rather than the compositor's vblank.
+#
+# Usage: run-bench.sh <bindir> <label> <meshes> <reps> [--cold]
+#   --cold deletes pipeline_cache.bin before EACH run (cold-compile path).
+set -u
+BINDIR="$1"; LABEL="$2"; MESHES="$3"; REPS="$4"; COLD="${5:-}"
+cd "$BINDIR" || exit 1
+
+declare -A vals
+metrics="setup_ms fps frame_mean_ms frame_p50_ms frame_p99_ms peak_rss_kb"
+for m in $metrics; do vals[$m]=""; done
+
+for ((i=0;i<REPS;i++)); do
+  [ "$COLD" = "--cold" ] && rm -f pipeline_cache.bin
+  out=$(VK_LOADER_LAYERS_DISABLE='~all~' CRAFTER_PRESENT_IMMEDIATE=1 \
+        BENCH_WARMUP=200 BENCH_FRAMES=2000 BENCH_MESHES="$MESHES" \
+        ./SponzaBench 2>/dev/null)
+  for m in $metrics; do
+    v=$(echo "$out" | awk -v k="$m" '$1=="BENCH" && $2==k {print $3}')
+    vals[$m]="${vals[$m]} $v"
+  done
+done
+
+med() { tr ' ' '\n' | grep -v '^$' | sort -n | awk '{a[NR]=$1} END{print a[int((NR+1)/2)]}'; }
+lo()  { tr ' ' '\n' | grep -v '^$' | sort -n | head -1; }
+hi()  { tr ' ' '\n' | grep -v '^$' | sort -n | tail -1; }
+
+echo "## $LABEL  (meshes=$MESHES reps=$REPS ${COLD})"
+printf "%-15s %12s %12s %12s\n" metric median min max
+for m in $metrics; do
+  echo -n "$(printf '%-15s' "$m") "
+  printf "%12s %12s %12s\n" \
+    "$(echo "${vals[$m]}" | med)" "$(echo "${vals[$m]}" | lo)" "$(echo "${vals[$m]}" | hi)"
+done
+echo