router: CAKE shaper on WAN egress at 63M to kill upstream bufferbloat

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

services/router.nix

@@ -122,6 +122,27 @@ in
             # false = keep the lease and re-REQUEST it (INIT-REBOOT).
             SendRelease = false;
           };
+
+          # CAKE shaper on WAN egress. eno1 carries fq_codel at 1Gbit line
+          # rate, so it never actually engages: the uplink is ~70Mbit, which
+          # means the real queue forms upstream in TalkTalk's kit where we
+          # can't touch it. Shaping just under measured line rate pulls that
+          # queue down here, where CAKE's per-flow fairness keeps sparse
+          # latency-sensitive traffic (games, VoIP) from sitting behind a bulk
+          # upload. Measured by router-speedtest: 8.73-9.52 MB/s up, i.e.
+          # ~70Mbit floor; 63M is 90% of that.
+          #
+          # NAT=true so flow isolation resolves real LAN hosts rather than
+          # lumping the whole house behind one post-NAT address. Kernel default
+          # FlowIsolationMode (triple-isolate) is host-aware, so this matters.
+          #
+          # Egress only: 500Mbit down is far harder to saturate and shaping it
+          # would need an IFB device for not much gain.
+          cakeConfig = {
+            Parent = "root";
+            Bandwidth = "63M";
+            NAT = true;
+          };
         };
         "20-lan" = {
           matchConfig.Name = "eth0";