llm-inference-multimodel: stop pre-existing Gemma service before Phase 4 starts new instances

ansible/roles/llm-inference-multimodel/tasks/verify.yml

@@ -10,6 +10,31 @@
 #              systemd services" intent for durability, not just this-session.
 # ------------------------------------------------------------------------------
 
+# BUGFIX (found in production): this file used to go straight from Phase 2's
+# "units deployed, nothing running" state into starting/enabling both new
+# services, without first stopping the pre-existing single-model Gemma
+# llama-server that Phase 0 (discover.yml) confirmed is running on the same
+# GPU. Plan §1's VRAM math (aux ~Xgb + toolcall ~Ygb ≈ ~23.5GB / 24GB) only
+# holds if that card isn't ALSO holding the old Gemma model's VRAM at the
+# same time. Running --tags verify (or the full playbook) against a host
+# where Gemma was still up meant starting both new instances on top of it —
+# a near-guaranteed OOM-kill, not just a "risk" the old §6 debug message
+# warned about after the fact.
+#
+# Fix: if discover.yml found a systemd-managed Gemma unit
+# (llm_existing_gemma_unit_found), stop it here, before either new service
+# starts. This only stops the unit — it does not disable it, so plan §6's
+# rollback ("systemctl start llama-server-gemma to fully revert") still
+# works unchanged. If no such unit was found, this is a no-op and the
+# original WARNING from discover.yml (any Gemma process running outside
+# systemd) still stands as a human decision point.
+- name: Stop pre-existing Gemma llama-server before starting new instances (avoid double VRAM usage / OOM)
+  ansible.builtin.systemd:
+    name: "{{ llm_existing_gemma_service_name_guess }}"
+    state: stopped
+  become: true
+  when: llm_existing_gemma_unit_found | default(false)
+
 - name: Enable llama-server-aux and start/restart based on Phase 2 unit-content change
   ansible.builtin.systemd:
     name: "{{ llm_aux_service_name }}"