Key under which a MiddlewareFunction carries its RouteSecurityMetadata.

Created with Symbol.for, not Symbol(), so two copies of this package in one process resolve the same key — the failure mode a locally-constructed symbol would produce silently, since every read would simply miss.

Prefer withSecurityMetadata and securityMetadataOf over touching this directly; the symbol is exported so a guard outside @setu-ts/auth-plugin can be branded too.