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/usr/local/bin/bee-network.sh

@@ -1,9 +1,12 @@
 #!/bin/sh
-# bee-network.sh — bring up all physical interfaces via DHCP (non-blocking)
+# bee-network.sh — bring up all physical network interfaces via DHCP
+# Unattended: runs silently, logs results, never blocks.
 
 LOG_PREFIX="bee-network"
+
 log() { echo "[$LOG_PREFIX] $*"; }
 
+# find physical interfaces: exclude lo and virtual (docker/virbr/veth/tun/tap)
 interfaces=$(ip -o link show \
     | awk -F': ' '{print $2}' \
     | grep -v '^lo$' \
@@ -16,9 +19,13 @@ if [ -z "$interfaces" ]; then
 fi
 
 for iface in $interfaces; do
-    ip link set "$iface" up 2>/dev/null || { log "WARN: failed to bring up $iface"; continue; }
-    udhcpc -i "$iface" -b -t 0 -T 3 >/dev/null 2>&1 &
-    log "dhcp started for $iface"
+    log "bringing up $iface"
+    ip link set "$iface" up 2>/dev/null || { log "WARN: could not bring up $iface"; continue; }
+
+    # DHCP in background: -b forks if no immediate lease, & ensures non-blocking always.
+    # -t 0: unlimited retries, -T 3: 3s per attempt. No -q: stay running to renew lease.
+    udhcpc -i "$iface" -b -t 0 -T 3 &
+    log "DHCP started for $iface (pid $!)"
 done
 
 log "done"