policyOverlay
Wrap base in a gossip-relay layer under an explicit topology policy, with zero recompute jitter, started on the receiver scope. The generalization of starOverlay (which is just policyOverlay(base, FullFanout, …)): the same started-GossipSeam relay interior, but the dissemination shape is whatever TopologyPolicy the caller passes — FullFanout (the hub star), RandomKRegular (the k-regular partial mesh), or TwoTier (a federated server core + client periphery). Broadcasts flood along the policy's active view; Seam.sendTo passes through unwrapped — the tested unicast invariant that keeps per-recipient traffic off the flood path is independent of the shape.
Zero jitter is deliberate, not tuning, and it carries across every shape passed here — the two shipped structural/full policies (FullFanout, TwoTier) are deterministic functions of the roster (and, for TwoTier, the core + attachment), so the anti-lockstep jitter buys them nothing and only opens a window where a freshly admitted peer is missing from the flood targets — a broadcast in that window would skip the peer and leave a per-origin seq gap it then reorder-holds behind (#1309). The view recomputes synchronously with each roster change instead. (A caller wanting jittered recompute for an isotropic random mesh constructs GossipSeam directly; this overlay is the deterministic-shape composition path.)
Parameters
the local endpoint this node relays over — a hub's fanout seam, a spoke's link to the hub, or a server/client's link into a two-tier graph.
the dissemination shape — who this node eager-floods broadcasts to. Required: the overlay's whole job is to name a shape, so there is no default (unlike GossipSeam, whose default is a random mesh).
RNG for overlay bookkeeping (and for a policy that owns selection randomness, e.g. RandomKRegular); tests inject a seeded instance.
Clock for the overlay's per-neighbour liveness detectors. Required — no wall-clock default, so a virtual-time caller can never silently fall through to the system clock. Production callers pass { kotlin.time.Clock.System.now() }.