node/peering: add the sans-IO peering reconciler core (unwired)

Introduce PeeringReconciler, the synchronous decision core for peer
desired-state, with its input/action vocabulary (Gate, Budget, Observed,
Candidate, DiscoveryPools, Policy, PeeringAction) and the Peering owner
struct. reconcile() computes connect/retry intents across four layers:
the auto-connect mandatory floor, the ceiling-only overlay enqueue, the
opportunistic transport-neighbor growth, and the node.limits ceiling
enforced inline. It mirrors the supervisor sans-IO shape: no I/O, no
clock reads (time enters as a parameter), no runtime handles.

The core owns its own cross-attempt retry schedule so escalating backoff
survives the fresh connection created per re-dial. Gate-guarded reflexes
(on_handshake_timeout / on_link_dead) reproduce the current backoff math
and no-op while draining. Overlay is strictly ceiling-only (no peer-count
set-point). Disconnect is defined but never emitted (no shedding).

Unwired here: the driver still runs the imperative peering methods and
Node holds the live retry map. 11 unit tests drive the pure core with
synthetic inputs. No behavior change.

src/node/peering/mod.rs

@@ -10,3 +10,6 @@
 //! persist in the reconciler, not per-connection.
 
 pub(in crate::node) mod retry;
+
+#[allow(dead_code)] // Wired by the driver cutover in the following commits; unused until then.
+pub(in crate::node) mod reconcile;