docs(test): add OIDC/MSC3861 test section + local MAS dev loop

LOTUS_TESTING.md section N (N1-N6): OIDC login flow, session-persist-on-reload,
token refresh, logout revocation, account-management link, and the non-OIDC
regression check. Backed by dev/oidc-test/ — a runnable local Matrix
Authentication Service + Synapse(msc3861) loop (compose skeleton, the Synapse
experimental_features delta, and the public/config.json override) so the flow
can be verified without a mozilla.org tester.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dev/oidc-test/synapse-msc3861.yaml

@@ -0,0 +1,16 @@
+# Synapse experimental-features delta to delegate auth to a local MAS (MSC3861).
+# Merge this into your test homeserver.yaml. The client_secret + admin_token MUST
+# match the MAS config (clients[].client_secret and matrix.secret respectively).
+experimental_features:
+  msc3861:
+    enabled: true
+    issuer: http://localhost:8090/
+    client_id: "0000000000000000000SYNAPSE"
+    client_auth_method: client_secret_basic
+    client_secret: "REPLACE_WITH_A_SHARED_CLIENT_SECRET"
+    admin_token: "REPLACE_WITH_A_LONG_SHARED_ADMIN_TOKEN"
+    account_management_url: "http://localhost:8090/account"
+
+# With msc3861 enabled, Synapse disables its own password/SSO login and advertises
+# `m.authentication` in /.well-known/matrix/client — which is exactly what the
+# Lotus client's getOidcIssuer() reads to switch into the OIDC flow.