chore: archive observability-service-registry, track pi-map artifacts

- Archive the completed observability-service-registry SDD change into
  openspec/changes/archive/ (delivered across 5 slices; only
  jellyfin-service-registry remains active).
- Stop ignoring .pi-map.md / .pi-map.index.md so the navigation maps are
  versioned alongside the code, and add the regenerated map pairs repo-wide.

frontend/public/.pi-map.index.md

@@ -0,0 +1,20 @@
+# frontend/public (index)
+dir: frontend/public
+
+## role
+Provides static public assets for the frontend web application, including brand identity graphics and reusable UI icons.
+## parent
+index: frontend/.pi-map.index.md
+map: frontend/.pi-map.md
+## children
+-
+## files
+- favicon.svg
+- icons.svg
+## links
+index: frontend/public/.pi-map.index.md
+map: frontend/public/.pi-map.md
+## workflows
+-
+## dirty
+-