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>

build/config/rpi-kernel-config.fragment

@@ -0,0 +1,69 @@
+# KubeSolo OS — Raspberry Pi kernel config overrides
+# Applied on top of bcm2711_defconfig (Pi 4) or bcm2712_defconfig (Pi 5)
+# These ensure container runtime support is enabled.
+
+# cgroup v2 (mandatory for containerd/runc)
+CONFIG_CGROUPS=y
+CONFIG_CGROUP_CPUACCT=y
+CONFIG_CGROUP_DEVICE=y
+CONFIG_CGROUP_FREEZER=y
+CONFIG_CGROUP_SCHED=y
+CONFIG_CGROUP_PIDS=y
+CONFIG_MEMCG=y
+CONFIG_CGROUP_BPF=y
+CONFIG_CFS_BANDWIDTH=y
+
+# BPF (required for cgroup v2 device control)
+CONFIG_BPF=y
+CONFIG_BPF_SYSCALL=y
+
+# Namespaces (mandatory for containers)
+CONFIG_NAMESPACES=y
+CONFIG_NET_NS=y
+CONFIG_PID_NS=y
+CONFIG_USER_NS=y
+CONFIG_UTS_NS=y
+CONFIG_IPC_NS=y
+
+# Device management
+CONFIG_DEVTMPFS=y
+CONFIG_DEVTMPFS_MOUNT=y
+
+# Filesystem
+CONFIG_OVERLAY_FS=y
+CONFIG_SQUASHFS=y
+CONFIG_EXT4_FS=y
+CONFIG_VFAT_FS=y
+
+# Networking
+CONFIG_BRIDGE=m
+CONFIG_NETFILTER=y
+CONFIG_NF_CONNTRACK=m
+CONFIG_NF_NAT=m
+CONFIG_NF_TABLES=m
+CONFIG_VETH=m
+CONFIG_VXLAN=m
+
+# Security: AppArmor + Audit
+CONFIG_AUDIT=y
+CONFIG_AUDITSYSCALL=y
+CONFIG_SECURITY=y
+CONFIG_SECURITYFS=y
+CONFIG_SECURITY_NETWORK=y
+CONFIG_SECURITY_APPARMOR=y
+CONFIG_DEFAULT_SECURITY_APPARMOR=y
+
+# Security: seccomp
+CONFIG_SECCOMP=y
+CONFIG_SECCOMP_FILTER=y
+
+# Crypto (image verification)
+CONFIG_CRYPTO_SHA256=y
+
+# Disable unnecessary subsystems for edge appliance
+# CONFIG_SOUND is not set
+# CONFIG_DRM is not set
+# CONFIG_MEDIA_SUPPORT is not set
+# CONFIG_WIRELESS is not set
+# CONFIG_BT is not set
+# CONFIG_NFC is not set