fix: NAM CPU display now responds to bypass

The CPU label in the UI was showing psutil.cpu_percent(interval=None)
which returns total system CPU — barely moves and doesn't change when
the NAM model is bypassed.

Fix:
- Added _calc_nam_cpu() that computes NAM engine CPU from the engine's
  own per-block inference timing (avg_inference_ms) vs the JACK callback
  interval. When the NAM block is bypassed in the chain, returns 0.
- Added 'nam_cpu' field to /api/state response
- Status bar shows NAM CPU % next to VU meters (drops to 0 on bypass)
- Settings CPU row shows NAM CPU with system CPU in dim text beside it
- Changed psutil.cpu_percent(interval=None) → interval=0.2 for a real
  system CPU reading

src/web/server.py

@@ -2490,12 +2490,40 @@ iface eth0 inet static
         try:
             import psutil
             return {
-                "cpu_percent": psutil.cpu_percent(interval=None),
+                "cpu_percent": psutil.cpu_percent(interval=0.2),
                 "sample_rate": SAMPLE_RATE if hasattr(self, 'deps') and self.deps.pipeline else 48000,
             }
         except Exception:
             return {"cpu_percent": 0, "sample_rate": 48000}
 
+    def _calc_nam_cpu(self, nam, pipeline) -> float:
+        """Estimate NAM engine CPU load as a percentage of real-time.
+
+        Uses the engine's own per-block inference timing. When the NAM
+        block is bypassed or the engine isn't loaded, returns 0.
+        """
+        if not nam or not nam.is_loaded:
+            return 0.0
+        # Check if the NAM block is bypassed in the pipeline chain
+        if pipeline and hasattr(pipeline, '_chain'):
+            for entry in pipeline._chain:
+                if entry.get("fx_type") and entry["fx_type"].value in ("nam_amp", "nam"):
+                    if entry.get("bypass") or not entry.get("enabled", True):
+                        return 0.0
+                    break
+        avg_ms = nam.avg_inference_ms
+        if avg_ms <= 0:
+            return 0.0
+        try:
+            period = self.deps.audio_system.config.latency_profile["period"]
+            rate = self.deps.audio_system.config.latency_profile["rate"]
+            callback_ms = (period / rate) * 1000.0
+            if callback_ms <= 0:
+                return 0.0
+            return round(min(100.0, (avg_ms / callback_ms) * 100.0), 1)
+        except Exception:
+            return 0.0
+
     # ── State gathering ──────────────────────────────────────────────
 
     def _gather_channel_state(self, channel: str) -> dict[str, Any]:
@@ -2616,6 +2644,7 @@ iface eth0 inet static
             ),
             # System stats
             "cpu_percent": self._gather_system_stats().get("cpu_percent", 0),
+            "nam_cpu": self._calc_nam_cpu(nam, pl),
             "sample_rate": SAMPLE_RATE,
             "wifi": self._gather_wifi_state(),
             "bluetooth": self._gather_bt_state(),