Add Pi signage player phase 1 artifacts

apps/fc-signage-pi-player/README.md

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+# FlowerCore Signage Pi Player
+
+Phase 1 Raspberry Pi signage player packaging for Chromium kiosk deployments.
+This bundle is intentionally air-gap friendly: systemd units, shell scripts,
+udev rules, and Chromium managed policy are all checked into the repo and are
+installed by `FlowerCore.Puppet`.
+
+## Scope
+
+- Bootstrap a stable node identity and mTLS client certificate.
+- Launch Chromium in kiosk mode against `FC.Signage.Web` player routes.
+- Restart the kiosk on HDMI hotplug.
+- Renew mTLS certificates daily when fewer than 30 days remain.
+- Detect display capabilities at boot, daily, and on HDMI hotplug.
+
+Phase 2 native Avalonia rendering is documented separately in Notes and remains
+deferred.