Distinguish an upstream context (passed by a parent middleware) from a host-supplied platform argument (an env / connection-info object the runtime puts in the same positional slot). The symbol marker set by seedContext flows by reference through every { ...upstream } merge (spread copies enumerable own symbols), so checking for it is reliable across the stack.
Public so a host embedding the engine (e.g. @supabase/server) can make the same distinction before calling seedContext: reseeding with an upstream context would stash it as the platform env, clobbering the real bindings captured earlier.