Append NVIDIA params to GRUB cmdline instead of overwriting

Previously the task hardcoded GRUB_CMDLINE_LINUX to a fixed string
containing nymph's rd.luks.uuid, rootflags and apparmor parameters,
clobbering whatever the host had. Switch to an idempotent
lineinfile-with-backrefs loop that appends nouveau.modeset=0 and
nvidia-drm.modeset=1 only when missing, preserving the existing
kernel cmdline. grub-mkconfig now runs only when the cmdline changed.

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

ansible/roles/profile_desktop_host/tasks/nymph.yml

@@ -1,16 +1,21 @@
 ---
-- name: Configure GRUB kernel parameters for NVIDIA hybrid graphics
+- name: Ensure NVIDIA kernel parameters are present in GRUB cmdline
   tags: [packages, nvidia]
   ansible.builtin.lineinfile:
     path: /etc/default/grub
-    regexp: '^GRUB_CMDLINE_LINUX='
-    line: 'GRUB_CMDLINE_LINUX="rd.luks.uuid=1e15d159-5d05-4a1f-9639-ac200dff9f9c rootflags=subvol=@ apparmor=1 security=apparmor nouveau.modeset=0 nvidia-drm.modeset=1"'
-    state: present
+    backrefs: true
+    regexp: '^(GRUB_CMDLINE_LINUX="(?!.*{{ item | regex_escape }}).*)"$'
+    line: '\1 {{ item }}"'
+  loop:
+    - nouveau.modeset=0
+    - nvidia-drm.modeset=1
+  register: nymph_grub_cmdline
 
 - name: Regenerate GRUB configuration
   tags: [packages, nvidia]
   ansible.builtin.command: grub-mkconfig -o /boot/grub/grub.cfg
   changed_when: true
+  when: nymph_grub_cmdline is changed
 
 - name: Configure NVIDIA power management for hybrid graphics
   tags: [packages, nvidia]