[core-server-kit] Can't build the `virtualbox-modules` with `debian-sources-6.16.3`

In file included from SUPDrv.c:43:
SUPDrvInternal.h:47:10: fatal error: VBox/cdefs.h: No such file or directory
   47 | #include <VBox/cdefs.h>
      |          ^~~~~~~~~~~~~~
compilation terminated.
In file included from linux/SUPDrv-linux.c:42:
linux/../SUPDrvInternal.h:47:10: fatal error: VBox/cdefs.h: No such file or directory
   47 | #include <VBox/cdefs.h>
      |          ^~~~~~~~~~~~~~
compilation terminated.
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: SUPDrv.o] Error 1
make[5]: *** Waiting for unfinished jobs....
In file included from SUPDrvSem.c:43:
SUPDrvInternal.h:47:10: fatal error: VBox/cdefs.h: No such file or directory
   47 | #include <VBox/cdefs.h>
      |          ^~~~~~~~~~~~~~
compilation terminated.
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: linux/SUPDrv-linux.o] Error 1
In file included from SUPDrvGip.c:43:
SUPDrvInternal.h:47:10: fatal error: VBox/cdefs.h: No such file or directory
   47 | #include <VBox/cdefs.h>
      |          ^~~~~~~~~~~~~~
compilation terminated.
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: SUPDrvSem.o] Error 1
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: SUPDrvGip.o] Error 1
In file included from SUPDrvTracer.c:43:
SUPDrvInternal.h:47:10: fatal error: VBox/cdefs.h: No such file or directory
   47 | #include <VBox/cdefs.h>
      |          ^~~~~~~~~~~~~~
compilation terminated.
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: SUPDrvTracer.o] Error 1
combined-agnostic1.c:38:10: fatal error: internal/iprt.h: No such file or directory
   38 | #include "internal/iprt.h"
      |          ^~~~~~~~~~~~~~~~~
compilation terminated.
SUPLibAll.c:41:10: fatal error: VBox/sup.h: No such file or directory
   41 | #include <VBox/sup.h>
      |          ^~~~~~~~~~~~
compilation terminated.
common/string/strformatrt.c:42:10: fatal error: iprt/string.h: No such file or directory
   42 | #include <iprt/string.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: combined-agnostic1.o] Error 1
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: SUPLibAll.o] Error 1
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: common/string/strformatrt.o] Error 1
combined-agnostic2.c:38:10: fatal error: internal/iprt.h: No such file or directory
   38 | #include "internal/iprt.h"
      |          ^~~~~~~~~~~~~~~~~
compilation terminated.
combined-os-specific.c:38:10: fatal error: the-linux-kernel.h: No such file or directory
   38 | #include "the-linux-kernel.h"
      |          ^~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: combined-agnostic2.o] Error 1
make[5]: *** [/usr/src/linux-debian-sources-6.16.3_p1/scripts/Makefile.build:292: combined-os-specific.o] Error 1
make[4]: *** [/usr/src/linux-debian-sources-6.16.3_p1/Makefile:2027: .] Error 2
make[3]: *** [/usr/src/linux-debian-sources-6.16.3_p1/Makefile:260: __sub-make] Error 2
make[3]: Leaving directory '/var/tmp/portage/app-emulation/virtualbox-modules-7.1.8/work/virtualbox-modules-7.1.8/vboxdrv'
make[2]: *** [Makefile:260: __sub-make] Error 2
make[2]: Leaving directory '/usr/src/linux-debian-sources-6.16.3_p1'
make[1]: *** [/var/tmp/portage/app-emulation/virtualbox-modules-7.1.8/work/virtualbox-modules-7.1.8/vboxdrv/Makefile-footer.gmk:145: vboxdrv] Error 2
make[1]: Leaving directory '/var/tmp/portage/app-emulation/virtualbox-modules-7.1.8/work/virtualbox-modules-7.1.8/vboxdrv'
make: *** [Makefile:78: vboxdrv] Error 2

Start with the virtualbox-modules-7.1.8 vboxdrv build output and inspect vboxdrv/Makefile-footer.gmk, SUPDrvInternal.h, and the reported missing include paths. Compare the module build inputs with the available VirtualBox source headers, then rerun the package build against debian-sources-6.16.3. Done means the virtualbox-modules package builds without the reported missing-header errors.