perf(data-plane): reduce per-packet synchronization overhead (#2453)

* perf(stats): avoid per-update clock reads

Perf profiles show quanta::get_now consuming 2.9-4.3% of data-plane
CPU because every counter update refreshes a high-resolution timestamp.

Track metric activity with the existing 60-second cleanup cadence instead.
Relaxed 32-bit epochs preserve the three-minute retention window, support
32-bit targets, and remove repeated clock reads from packet processing.

* perf(data-plane): reduce per-packet synchronization

Perf profiles showed per-packet config Arc cloning, bounded-channel
permit futures, duplicate peer lookups, and default connection UUID
lookups consuming CPU in both TCP and UDP data paths.

Borrow stable config snapshots, use nonblocking channel fast paths with
the existing backpressure fallback, reuse direct peer lookups, and cache
the selected connection while preserving close and reselection behavior.

Add focused tests for channel backpressure and cached connection
invalidation.

* fix(peer): serialize default connection cache updates

The profile-guided default connection cache could republish a connection
after the close task removed it, leaving a stale cache while another
connection remained live.

Serialize only cache-miss selection/publication and connection removal.
The per-packet cache-hit path remains lock-free, while close and selection
can no longer race to resurrect a removed connection.

easytier-core/src/peers/context.rs

@@ -763,7 +763,8 @@ impl PeerContext for CorePeerContext {
     }
 
     fn packet_policy(&self) -> PeerPacketPolicy {
-        PeerPacketPolicy::from_flags(&self.snapshot().flags)
+        self.config
+            .with_snapshot(|snapshot| PeerPacketPolicy::from_flags(&snapshot.peer.flags))
     }
 
     fn host_routing_policy(&self) -> HostRoutingPolicy {
@@ -792,13 +793,16 @@ impl PeerContext for CorePeerContext {
     }
 
     fn ipv4(&self) -> Option<Ipv4Inet> {
-        self.snapshot()
-            .runtime
-            .core
-            .routes
-            .ipv4
-            .as_ref()
-            .and_then(config_ipv4)
+        self.config.with_snapshot(|snapshot| {
+            snapshot
+                .peer
+                .runtime
+                .core
+                .routes
+                .ipv4
+                .as_ref()
+                .and_then(config_ipv4)
+        })
     }
 
     fn ipv6(&self) -> Option<Ipv6Inet> {