The deployed signed chain: AudioProxy.Plugs.SignedChain and then the action.

The checks ahead of the action, their order and the assigns contract between them live in AudioProxy.Plugs.SignedChain, which the test pipelines mount too — this module is that unit plus the production action options, and the test mountings differ from it only in theirs.

A new check goes in SignedChain, never here. A plug added between the two below runs in production and in neither test mounting, which is the divergence the extraction removed, reinstated in one line. Nothing prevents it structurally: the seam is "everything ahead of the action", and moving it would mean making the action mountable through the unit.

Mounted by the router for GET /:sig/*rest; /health stays outside it, so an unsigned liveness check never touches signature verification.

The action is the seam, and there are two behind it: the streaming render and the info probe. Both endpoints in §2 are the same route and the same checks, so which one a request reached is a fact the chain discovers rather than the router — AudioProxy.Plugs.ParseOptions decides it and AudioProxy.Plugs.Action acts on it.