chore(bench): add client bench harness and Phase 2 handoff

The client now boots cleanly on the bench machine (the CAC spawn-key
crash was fixed in 8e8483d). Add the harness used to drive bench scenes
and capture frametimes, the bench-world generation workflow, and a
self-contained handoff brief. All files live under analysis/ (packwizignored),
so nothing player-facing changes.

- harness/rcon.py: minimal RCON client for the temp worldgen server
- harness/run_scene.py: apply variant -> launch Prism under MangoHud ->
  xdotool scene commands -> p50/p95/p99/1%-low from the CSV
- HANDOFF.md: state, gotchas, next steps
- build.py: disable DH autoUpdater for bench variants (suppress modal)

analysis/client-perf/proposed-configs/build.py

@@ -182,7 +182,11 @@ HIGH = {  # strong GPU, but still a 4C/8T CPU (true of every fleet machine)
 BENCH = {
     "options.txt": {"maxFps": 260, "enableVsync": "false", "renderDistance": 8,
                     "simulationDistance": 8},
-    "config/DistantHorizons.toml": {(DH_W, "generatorPlan"): "DISABLED"},
+    "config/DistantHorizons.toml": {
+        (DH_W, "generatorPlan"): "DISABLED",
+        # Harness: suppress DH's "new update available" modal on every launch.
+        ("client.advanced.autoUpdater", "enableAutoUpdater"): False,
+    },
 }