Add the compose_stack role, inventory and playbooks

One role that syncs a payload and brings the stack up, configured entirely
through stack_* variables, plus six playbooks that each demonstrate one part
of that contract. Every playbook runs standalone and is tagged with its
stack name, so site.yml --tags <stack> works.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

ansible/requirements.yml

@@ -0,0 +1,8 @@
+---
+collections:
+  # docker_compose_v2 — bringing stacks up.
+  - name: community.docker
+    version: ">=3.10.0"
+  # synchronize — rsyncing stack payloads to the hosts.
+  - name: ansible.posix
+    version: ">=1.5.0"