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/harness/rcon.py

@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+"""Minimal Minecraft RCON client (stdlib only).
+
+Usage: python3 rcon.py <host> <port> <password> <command> [command ...]
+Exit non-zero if the server reports a failure.
+"""
+import socket
+import struct
+import sys
+
+
+class Rcon:
+    def __init__(self, host: str, port: int, password: str, timeout: float = 10.0):
+        self.host, self.port, self.password = host, port, password
+        self.timeout = timeout
+        self._id = 0
+
+    def _send(self, sock: socket.socket, ptype: int, body: str) -> int:
+        self._id += 1
+        rid = self._id
+        payload = struct.pack("<ii", rid, ptype) + body.encode("utf-8") + b"\x00\x00"
+        sock.sendall(struct.pack("<i", len(payload)) + payload)
+        return rid
+
+    def _recv(self, sock: socket.socket):
+        raw = b""
+        while len(raw) < 4:
+            chunk = sock.recv(4 - len(raw))
+            if not chunk:
+                raise ConnectionError("RCON connection closed")
+            raw += chunk
+        (length,) = struct.unpack("<i", raw)
+        body = b""
+        while len(body) < length:
+            chunk = sock.recv(length - len(body))
+            if not chunk:
+                raise ConnectionError("RCON connection closed")
+            body += chunk
+        rid, ptype = struct.unpack("<ii", body[:8])
+        return rid, ptype, body[8:-2].decode("utf-8", "replace")
+
+    def command(self, cmd: str) -> str:
+        with socket.create_connection((self.host, self.port), timeout=self.timeout) as sock:
+            sock.settimeout(self.timeout)
+            rid = self._send(sock, 3, self.password)
+            rid2, _, _ = self._recv(sock)
+            if rid2 == -1:
+                raise PermissionError("RCON auth failed")
+            self._send(sock, 2, cmd)
+            _, _, text = self._recv(sock)
+            return text
+
+
+def main(argv):
+    if len(argv) < 5:
+        print(__doc__)
+        return 2
+    host, port, password = argv[1], int(argv[2]), argv[3]
+    rc = 0
+    for cmd in argv[4:]:
+        try:
+            out = Rcon(host, port, password).command(cmd)
+        except Exception as exc:  # noqa: BLE001 - CLI surface
+            print(f"! {cmd}: {exc}", file=sys.stderr)
+            rc = 1
+            continue
+        if out.strip():
+            print(out.strip())
+        lowered = out.lower()
+        if "unknown" in lowered or "failed" in lowered or "error" in lowered:
+            rc = 1
+    return rc
+
+
+if __name__ == "__main__":
+    raise SystemExit(main(sys.argv))