refactor(ext-auth): generic sessions, shared annotation wiring, rename to flash-ext-auth-oidc

AuthMiddleware.install(ctx, config, source) now owns the annotation processor and
the flash.auth.policy key, so a second credential source gets annotation-driven
authorization without copying the wiring. The key is public: an extension that
contributes middleware can order itself around authentication.

OidcSession becomes Session in auth-core, carrying claims, an expiry and an opaque
attribute map. OpenID Connect keeps its access, id and refresh tokens in that map
under its own keys, so renewal stays its business and core has no OAuth2 vocabulary
in it. isAccessTokenExpired() becomes isExpired(), with the 30s eager-renewal
window it always had and now a test for it.

flash-ext-oidc is renamed flash-ext-auth-oidc, matching cache-core/cache-caffeine
and data-core/data-hibernate.

flash-extensions/flash-ext-mcp/docs/keycloak.md

@@ -1,7 +1,7 @@
 # Keycloak cookbook
 
 `security.md` covers the OAuth2 mechanics `McpOidcIntegration` implements against any
-`flash-ext-oidc`-compatible provider. This is the Keycloak-specific setup: the exact Admin
+`flash-ext-auth-oidc`-compatible provider. This is the Keycloak-specific setup: the exact Admin
 Console configuration for a working MCP OAuth2 flow with open Dynamic Client Registration
 (DCR) — no pre-registered clients, any MCP client self-registers on first connect.