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/tests/.pi-map.md

@@ -0,0 +1,20 @@
+# frontend/tests
+dir: frontend/tests
+
+index: frontend/tests/.pi-map.index.md
+
+## role
+Test suite validating frontend utility functions for user state management and UI model transformation.
+## files
+- userState.test.mjs | Tests user state utility functions for merging session activity with user data and resolving user selection by various identifiers | dep: node:assert/strict, node:test, ../src/userState.js
+- users.test.mjs | Tests the buildUserDrawerModel function that transforms user data into a UI drawer model with contact and permission details | dep: node:assert/strict, node:test, ../src/users.js
+## arch
+Node.js Mocha/ES module test pattern testing pure data transformation functions in isolation.
+## tags
+user, tests, data, node:assert, strict, node:test, src, js
+## symbols
+-
+## workflows
+-
+## dirty
+-