NVENC/NVDEC userspace path + nvidia-smi shortcut

- systemd-user environment generator and appsandbox-gpu wrapper both
  append /usr/lib/wsl/lib to LD_LIBRARY_PATH. ldconfig has the .so.1
  entries cached, but the 9P mount is read-only so the unversioned
  symlinks ldconfig wants to create can't be written. dlopen() of
  unversioned names (libcuda.so, libnvidia-encode.so, libnvcuvid.so)
  needs the path on LD_LIBRARY_PATH — without it ffmpeg's h264_nvenc /
  h264_cuvid plugins fail with 'Cannot load libcuda.so.1'.

- setup.sh: symlink /usr/lib/wsl/lib/nvidia-smi to /usr/local/bin so
  the bare command works without env tweaks. Created unconditionally
  because the symlink target is resolved at exec time — the file
  appears under the agent's 9P mount on first boot regardless of when
  the symlink was created.

src/backend_win/disk_util.c

@@ -2762,6 +2762,15 @@ static BOOL generate_ubuntu_setup_sh(const wchar_t *path, BOOL ssh_enabled)
         "fi\n"
         "ldconfig\n"
         "\n"
+        "# --- 7b. nvidia-smi on PATH ---\n"
+        "# WSL/GPU-PV ships nvidia-smi via the host 9P mount at\n"
+        "# /usr/lib/wsl/lib/nvidia-smi (only present when the VM has a\n"
+        "# GPU partition + the agent has mounted the share). The symlink\n"
+        "# is created unconditionally — symlink target is resolved at\n"
+        "# exec time, so it 'comes alive' once the agent mounts the share\n"
+        "# at first boot, with or without the file existing now.\n"
+        "ln -sf /usr/lib/wsl/lib/nvidia-smi /usr/local/bin/nvidia-smi\n"
+        "\n"
         "# --- 8. daemon-reload, enable the helper services, set graphical ---\n"
         "# (Helper binaries + service files are already installed by section 3.\n"
         "#  Clipboard has no system service — agent spawns it as a child.)\n"