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/inittab

@@ -0,0 +1,13 @@
+::sysinit:/sbin/openrc sysinit
+::sysinit:/sbin/openrc boot
+::wait:/sbin/openrc default
+
+# Autologin on tty1
+tty1::respawn:/sbin/agetty --autologin root --noclear tty1 linux
+tty2::respawn:/sbin/getty 38400 tty2
+tty3::respawn:/sbin/getty 38400 tty3
+
+ttyS0::respawn:/sbin/getty -L 115200 ttyS0 vt100
+
+::ctrlaltdel:/sbin/reboot
+::shutdown:/sbin/openrc shutdown