S3C-11127: POC-specific-code: install yarn for the node 22 that nvm just installed

Author: anurag4DSB

poc-demo/HANDOVER.md

@@ -451,7 +451,7 @@ showing the cure reads better than hiding the symptom.
 
 ```bash
 git clone --branch poc/S3C-11127-demo git@github.com:scality/backbeat.git && cd backbeat
-nvm install 22 && nvm use 22   # or any node 22 on PATH; 24 breaks native modules
+nvm install 22 && nvm use 22 && npm install -g yarn   # node 22 plus yarn 1 for that node; 24 breaks native modules
 yarn demo:install              # yarn install, node-gyp workaround folded in
 yarn demo:preflight            # docker, node, free ports, python for node-gyp
 yarn demo:up                   # builds the two local images if missing, then the stack

poc-demo/PLAYBOOK.md

@@ -44,8 +44,9 @@ From nothing to the demo. Each was executed on the build machine on
 # 1. clone the demo branch and enter it                             (11:05:45, 20 s)
 git clone --branch poc/S3C-11127-demo git@github.com:scality/backbeat.git && cd backbeat
 
-# 2. node 22 on PATH; 24 breaks the native modules and the install. With nvm:
-nvm install 22 && nvm use 22
+# 2. node 22 on PATH, then yarn 1 for that node (nvm keeps global packages per node version,
+#    so a fresh node 22 has no yarn); 24 breaks the native modules and the install.
+nvm install 22 && nvm use 22 && npm install -g yarn
 
 # 3. install, with the one macOS/node-gyp workaround folded in      (11:06:05, 60 s)
 yarn demo:install