Merge debug/prod into single ISO build, fix NVIDIA module loading

## ISO build consolidation
- Remove separate debug/prod split: overlay-debug/, build-debug.sh,
  mkimg.bee_debug.sh, genapkovl-bee_debug.sh all deleted
- Single overlay: iso/overlay/ (was overlay-debug content)
- Single build script: build.sh (SSH, TUI, NVIDIA, vendor tools, bee-release)
- Single mkimage profile: bee (with dropbear, dialog, strace, gcompat, etc.)

## NVIDIA fixes
- Modules now stored at /usr/local/lib/nvidia/ instead of
  /lib/modules/<kver>/extra/nvidia/ — modloop squashfs mounts over that
  path at boot making overlay content there inaccessible
- bee-nvidia init: load via insmod (absolute path), not modprobe
- bee-nvidia init: create libnvidia-ml.so.1/libcuda.so.1 symlinks in /usr/lib/
- build-nvidia-module.sh: always install linux-lts-dev (not conditional) —
  stale 6.6.x headers caused wrong-kernel modules that never loaded at runtime
- build-nvidia-module.sh: create soname symlinks in cache
- KERNEL_VERSION in VERSIONS updated 6.6 → 6.12
- gcompat added to ISO packages (nvidia-smi is a glibc binary on musl Alpine)

## Service ordering
- bee-audit: add `after bee-nvidia` so NVIDIA enrichment always succeeds

## New tooling
- iso/builder/smoketest.sh: SSH smoke test for post-boot ISO validation
- iso/builder/build-gpu-burn.sh: builds gpu_burn vendor binary (CUDA 12.8+)
- vendor/gpu_burn included automatically if placed in iso/vendor/

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

iso/overlay/etc/init.d/dropbear

@@ -0,0 +1,37 @@
+#!/sbin/openrc-run
+
+description="Dropbear SSH server"
+
+depend() {
+    need localmount
+    after bee-sshsetup
+    use logger
+}
+
+check_config() {
+    if [ ! -e /etc/dropbear/dropbear_rsa_host_key ]; then
+        einfo "Generating RSA host key..."
+        /usr/bin/dropbearkey -t rsa -f /etc/dropbear/dropbear_rsa_host_key
+    fi
+    if [ ! -e /etc/dropbear/dropbear_ecdsa_host_key ]; then
+        einfo "Generating ECDSA host key..."
+        /usr/bin/dropbearkey -t ecdsa -f /etc/dropbear/dropbear_ecdsa_host_key
+    fi
+    if [ ! -e /etc/dropbear/dropbear_ed25519_host_key ]; then
+        einfo "Generating ED25519 host key..."
+        /usr/bin/dropbearkey -t ed25519 -f /etc/dropbear/dropbear_ed25519_host_key
+    fi
+}
+
+start() {
+    check_config || return 1
+    ebegin "Starting dropbear"
+    /usr/sbin/dropbear ${DROPBEAR_OPTS}
+    eend $?
+}
+
+stop() {
+    ebegin "Stopping dropbear"
+    start-stop-daemon --stop --pidfile /var/run/dropbear.pid
+    eend $?
+}