node/peering: drive the mandatory-floor and retry mechanism through the reconciler

Route the auto-connect floor and the connection-retry mechanism through
the sans-IO reconciler instead of the imperative Node methods. A thin
peering driver (note_handshake_timeout / note_link_dead) centralizes the
reflex path with the gate guard and the already-connected check; the
per-tick retry-dial and the startup floor build reconciler inputs and
execute the emitted Connect intents. The three imperative retry methods
are removed and their call sites rerouted.

Wire the drain and startup gates. Entering a bounded drain now clears the
retry schedule and suppresses the peer-loss reconnect reflex (the drain
window runs under the Suspended gate), so the drain no longer fights a
reconnect for the peers it just closed. The startup floor runs under an
explicit Reconciling gate at the existing peer-connect seam, preserving
the current dial position.

Behavior-neutral except the intended drain change. Overlay discovery and
opportunistic transport-neighbor growth remain imperative for now; they
observe the relocated retry schedule and are cut over next. Unit tests
migrated to the driver API with assertions unchanged (1607).

src/node/peering/mod.rs

@@ -9,7 +9,6 @@
 //! connection is created per re-dial, so the escalating backoff count must
 //! 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 driver;
 pub(in crate::node) mod reconcile;
+pub(in crate::node) mod retry;