stage4: add missing pre-distkmerge controller case for distkernel builds

[reavessm]

When a stage4 spec sets boot/kernel/<name>/distkernel: yes, catalyst's stagebase.py:build_kernel() calls controller.sh pre-distkmerge. The stage4 controller had no case for this action, so it fell through to the default
exit 1, making distkernel stage4 builds impossible.

This patch adds the missing pre-distkmerge) case, wiring up the existing targets/support/pre-distkmerge.sh script which runs emerge --oneshot sys-kernel/dracut inside the chroot before the distributed kernel package is
merged.

[reavessm]

For reference, this was the error I was hitting:

19 Jun 2026 01:21:16 -00: ERROR   : CatalystError: cmd(['/usr/share/catalyst/targets/stage4/controller.sh', 'pre-distkmerge']) exited 1
ERROR:catalyst:CatalystError: cmd(['/usr/share/catalyst/targets/stage4/controller.sh', 'pre-distkmerge']) exited 1
19 Jun 2026 01:21:16 -00: ERROR   : Exception running action sequence build_kernel
Traceback (most recent call last):
  File "/usr/lib/python3.14/site-packages/catalyst/base/stagebase.py", line 38, in run_sequence
    func()
    ~~~~^^
  File "/usr/lib/python3.14/site-packages/catalyst/base/stagebase.py", line 1715, in build_kernel
    cmd([self.settings['controller_file'], 'pre-distkmerge'], env=self.env)
    ~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/site-packages/catalyst/support.py", line 66, in cmd
    raise CatalystError('cmd(%r) exited %s' % (args, ret),
                        print_traceback=False)
catalyst.support.CatalystError: cmd(['/usr/share/catalyst/targets/stage4/controller.sh', 'pre-distkmerge']) exited 1
ERROR:catalyst:Exception running action sequence build_kernel
Traceback (most recent call last):
  File "/usr/lib/python3.14/site-packages/catalyst/base/stagebase.py", line 38, in run_sequence
    func()
    ~~~~^^
  File "/usr/lib/python3.14/site-packages/catalyst/base/stagebase.py", line 1715, in build_kernel
    cmd([self.settings['controller_file'], 'pre-distkmerge'], env=self.env)
    ~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/site-packages/catalyst/support.py", line 66, in cmd
    raise CatalystError('cmd(%r) exited %s' % (args, ret),
                        print_traceback=False)
catalyst.support.CatalystError: cmd(['/usr/share/catalyst/targets/stage4/controller.sh', 'pre-distkmerge']) exited 1