feat: add security hardening, AppArmor, and ARM64 Raspberry Pi support (Phase 6)

Security hardening: bind kubeconfig server to localhost, mount hardening
(noexec/nosuid/nodev on tmpfs), sysctl network hardening, kernel module
loading lock after boot, SHA256 checksum verification for downloads,
kernel AppArmor + Audit support, complain-mode AppArmor profiles for
containerd and kubelet, and security integration test.

ARM64 Raspberry Pi support: piCore64 base extraction, RPi kernel build
from raspberrypi/linux fork, RPi firmware fetch, SD card image with 4-
partition GPT and tryboot A/B mechanism, BootEnv Go interface abstracting
GRUB vs RPi boot environments, architecture-aware build scripts, QEMU
aarch64 dev VM and boot test.

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

init/lib/90-kubesolo.sh

@@ -87,7 +87,7 @@ if [ -f "$KUBECONFIG_PATH" ]; then
 
     # Serve kubeconfig via HTTP on port 8080 using BusyBox nc
     (while true; do
-        printf "HTTP/1.1 200 OK\r\nContent-Type: text/yaml\r\nConnection: close\r\n\r\n" | cat - "$EXTERNAL_KC" | nc -l -p 8080 2>/dev/null
+        printf "HTTP/1.1 200 OK\r\nContent-Type: text/yaml\r\nConnection: close\r\n\r\n" | cat - "$EXTERNAL_KC" | nc -l -s 127.0.0.1 -p 8080 2>/dev/null
     done) &
 
     log_ok "Kubeconfig available via HTTP"