feat: build NVIDIA open kernel modules during ISO build

- build-nvidia-module.sh: downloads nvidia open-gpu-kernel-modules source,
  builds against linux-lts headers, extracts nvidia-smi from .run installer
- modules cached by driver version + kernel version (rebuild only on update)
- .ko files injected into ISO overlay at /lib/modules/<kver>/extra/nvidia/
- bee-nvidia init script loads nvidia/nvidia-modeset/nvidia-uvm at boot
- NVIDIA_DRIVER_VERSION=550.54.15 (Turing+, H100/A100 supported)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/overlay-debug/etc/init.d/bee-nvidia

@@ -0,0 +1,23 @@
+#!/sbin/openrc-run
+
+description="Bee: load NVIDIA kernel modules"
+
+depend() {
+    need localmount
+    before bee-audit-debug
+}
+
+start() {
+    ebegin "Loading NVIDIA modules"
+    # Run depmod so kernel can locate our modules in /lib/modules/.../extra/
+    depmod -a 2>/dev/null || true
+
+    for mod in nvidia nvidia-modeset nvidia-uvm; do
+        if modprobe "$mod" 2>/dev/null; then
+            einfo "loaded: $mod"
+        else
+            ewarn "failed to load: $mod"
+        fi
+    done
+    eend 0
+}