feat(adr-110): wire observe_csi_frame_arrival into CSI receive path

Iter 19 — without this call, iter 18's EMA fps tracking was dead code
because csi_fps_samples stayed 0 forever and mesh_aligned_us_for_csi_frame
always fell back to the 20 Hz constant.

In udp_receiver_task's parse_esp32_frame branch, replace the bare
last_frame_time assignment with NodeState::observe_csi_frame_arrival,
which computes dt vs last_frame_time, feeds update_csi_fps_ema (α=1/8),
bumps csi_fps_samples, and sets last_frame_time as a side effect (same
value the bare assignment did).

Effect: after ~5 CSI frames arrive from any node, mesh_aligned_us_for_csi_frame
returns interpolated timestamps using the node's actually-observed frame
rate instead of the 20 Hz default. Real bench rate was ~10 fps, so this
halves the per-frame timestamp error in §A0.12-style multistatic alignment.

cargo check -p wifi-densepose-sensing-server --no-default-features → green.

Co-Authored-By: claude-flow <ruv@ruv.net>

v2/crates/wifi-densepose-sensing-server/src/main.rs

@@ -4370,7 +4370,10 @@ async fn udp_receiver_task(state: SharedState, udp_port: u16) {
                     let adaptive_model_clone = s.adaptive_model.clone();
 
                     let ns = s.node_states.entry(node_id).or_insert_with(NodeState::new);
-                    ns.last_frame_time = Some(std::time::Instant::now());
+                    // ADR-110 iter 19 — feed the per-node fps EMA from real
+                    // CSI arrivals. The helper sets `last_frame_time` as a
+                    // side effect, so the previous bare assignment is gone.
+                    ns.observe_csi_frame_arrival(std::time::Instant::now());
 
                     // ADR-084 Pass 3: cluster-Pi novelty sensor.
                     // Score this frame's feature vector against the per-node