fix: kernel must use /sbin/init, not piCore's /init

Root cause of the 'Run /init as init process' -> immediate SIGSEGV panic on
the generic ARM64 boot: piCore64's rootfs ships a /init script at the rootfs
root, and the kernel's init search order picks /init over /sbin/init. piCore's
init then exec's something incompatible with our environment and segfaults.

Two fixes:
1. inject-kubesolo.sh now removes the upstream /init after replacing
   /sbin/init. This is the structural fix — the rootfs no longer has the
   conflicting entry-point.
2. grub-arm64.cfg passes init=/sbin/init explicitly. Belt-and-suspenders in
   case any future rootfs source re-introduces /init.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build/scripts/inject-kubesolo.sh

@@ -55,6 +55,15 @@ rm -f "$ROOTFS/sbin/init"
 cp "$PROJECT_ROOT/init/init.sh" "$ROOTFS/sbin/init"
 chmod +x "$ROOTFS/sbin/init"
 
+# Remove piCore/TC's /init at the rootfs root. The kernel's init search order
+# is /init -> /sbin/init -> ..., so leaving piCore's /init in place causes the
+# kernel to run that instead of ours, which then segfaults (it tries to do
+# piCore-specific TCE handling on a host that doesn't have it).
+if [ -e "$ROOTFS/init" ]; then
+    rm -f "$ROOTFS/init"
+    echo "    Removed upstream /init (kernel will fall through to /sbin/init)"
+fi
+
 # Init stages
 mkdir -p "$ROOTFS/usr/lib/kubesolo-os/init.d"
 for stage in "$PROJECT_ROOT"/init/lib/*.sh; do