port: give the port container a GPU path -- it never had one

Reported as "the port has low FPS". Godot 4 renders through Vulkan and this
launcher passed nothing through, so it fell back to lavapipe: software Vulkan,
correct and slow. The decoder's launcher has had this block for a long time;
the container that actually runs a renderer was the one without it.

Same three cases as the decoder, including the part worth repeating: passing
/dev/dri alone does NOT work for NVIDIA -- Mesa cannot drive the card and the
proprietary userspace lives outside the image. It needs the container toolkit.

The NOTE now prints the full repo-add sequence, because the package is not in
Ubuntu's default repos and `apt install nvidia-container-toolkit` on its own
fails with 'no installation candidate' -- which reads like the package is
wrong rather than the source being missing.

docker/port/sylph-port

@@ -114,6 +114,53 @@ docker_args() {
   fi
 
   [ -n "${ANTHROPIC_API_KEY:-}" ] && _out+=(-e "ANTHROPIC_API_KEY=$ANTHROPIC_API_KEY")
+
+  # ── GPU ──
+  # This block did not exist until 2026-09-01, and its absence was reported as
+  # a symptom rather than a cause: **the port agent reported low FPS.** Godot 4
+  # renders through Vulkan, and with nothing passed through it was falling back
+  # to lavapipe — software Vulkan, which is correct and slow. The decoder's
+  # launcher had this logic; this one never did, so the container that actually
+  # runs a renderer was the one without a GPU.
+  #
+  # Three distinct cases, and conflating them is how you end up believing you
+  # have hardware Vulkan while running llvmpipe:
+  #
+  #   NVIDIA  needs the NVIDIA Container Toolkit (`--gpus all`). Passing
+  #           /dev/dri alone does NOT work — Mesa cannot drive an NVIDIA card,
+  #           and the proprietary userspace lives outside the image.
+  #   Mesa    (AMD/Intel) works with a plain /dev/dri passthrough plus the
+  #           host's render/video GIDs.
+  #   neither software Vulkan (lavapipe): correct, and slow.
+  if [ "${SYLPH_VULKAN:-auto}" = "sw" ]; then
+    _out+=(-e SYLPH_VULKAN=sw)
+  elif command -v nvidia-smi >/dev/null 2>&1 && nvidia-smi -L >/dev/null 2>&1; then
+    if docker info --format '{{json .Runtimes}}' 2>/dev/null | grep -q nvidia; then
+      _out+=(--gpus all)
+    else
+      echo "==> NOTE: NVIDIA GPU found but the NVIDIA Container Toolkit is not" >&2
+      echo "    installed, so Godot falls back to lavapipe (software — correct," >&2
+      echo "    slow, and the reason for any low-FPS report). To enable it:" >&2
+      echo "      curl -fsSL https://nvidia.github.io/libnvidia-container/gpgkey \\" >&2
+      echo "        | sudo gpg --dearmor -o /usr/share/keyrings/nvidia-container-toolkit-keyring.gpg" >&2
+      echo "      curl -fsSL https://nvidia.github.io/libnvidia-container/stable/deb/nvidia-container-toolkit.list \\" >&2
+      echo "        | sed 's#deb https://#deb [signed-by=/usr/share/keyrings/nvidia-container-toolkit-keyring.gpg] https://#g' \\" >&2
+      echo "        | sudo tee /etc/apt/sources.list.d/nvidia-container-toolkit.list" >&2
+      echo "      sudo apt update && sudo apt install -y nvidia-container-toolkit" >&2
+      echo "      sudo nvidia-ctk runtime configure --runtime=docker" >&2
+      echo "      sudo systemctl restart docker" >&2
+      _out+=(-e SYLPH_VULKAN=sw)
+    fi
+  elif [ -e /dev/dri/renderD128 ]; then
+    _out+=(--device /dev/dri)
+    for g in render video; do
+      gid=$(getent group "$g" | cut -d: -f3 || true)
+      [ -n "$gid" ] && _out+=(--group-add "$gid")
+    done
+  else
+    _out+=(-e SYLPH_VULKAN=sw)
+  fi
+
   printf '%s\n' "${_out[@]}"
 }