feat: Phase 4 — Live, Vitals, Zones, MAT, Settings screens

LiveScreen: GaussianSplatWebView + gaussian-splats.html (Three.js 3D viz), LiveHUD
VitalsScreen: BreathingGauge, HeartRateGauge, MetricCard (Reanimated arcs)
ZonesScreen: FloorPlanSvg (SVG heatmap 20x20), ZoneLegend, useOccupancyGrid
MATScreen: MatWebView + mat-dashboard.html (pure-JS disaster response), AlertCard/List, SurvivorCounter
SettingsScreen: ServerUrlInput (URL validation + test), ThemePicker, RssiToggle

Verified: tsc 0 errors, jest passes

mobile/src/screens/MATScreen/index.tsx

@@ -0,0 +1,138 @@
+import { useEffect, useRef } from 'react';
+import { useWindowDimensions, View } from 'react-native';
+import { ConnectionBanner } from '@/components/ConnectionBanner';
+import { ThemedView } from '@/components/ThemedView';
+import { colors } from '@/theme/colors';
+import { spacing } from '@/theme/spacing';
+import { usePoseStream } from '@/hooks/usePoseStream';
+import { useMatStore } from '@/stores/matStore';
+import { type ConnectionStatus } from '@/types/sensing';
+import { Alert, type Survivor } from '@/types/mat';
+import { AlertList } from './AlertList';
+import { MatWebView } from './MatWebView';
+import { SurvivorCounter } from './SurvivorCounter';
+import { useMatBridge } from './useMatBridge';
+
+const isAlert = (value: unknown): value is Alert => {
+  if (!value || typeof value !== 'object') {
+    return false;
+  }
+
+  const record = value as Record<string, unknown>;
+  return typeof record.id === 'string' && typeof record.message === 'string';
+};
+
+const isSurvivor = (value: unknown): value is Survivor => {
+  if (!value || typeof value !== 'object') {
+    return false;
+  }
+
+  const record = value as Record<string, unknown>;
+  return typeof record.id === 'string' && typeof record.zone_id === 'string';
+};
+
+const resolveBannerState = (status: ConnectionStatus): 'connected' | 'simulated' | 'disconnected' => {
+  if (status === 'connecting') {
+    return 'disconnected';
+  }
+
+  return status;
+};
+
+export const MATScreen = () => {
+  const { connectionStatus, lastFrame } = usePoseStream();
+
+  const { survivors, alerts, upsertSurvivor, addAlert, upsertEvent } = useMatStore((state) => ({
+    survivors: state.survivors,
+    alerts: state.alerts,
+    upsertSurvivor: state.upsertSurvivor,
+    addAlert: state.addAlert,
+    upsertEvent: state.upsertEvent,
+  }));
+
+  const { webViewRef, ready, onMessage, sendFrameUpdate, postEvent } = useMatBridge({
+    onSurvivorDetected: (survivor) => {
+      if (isSurvivor(survivor)) {
+        upsertSurvivor(survivor);
+      }
+    },
+    onAlertGenerated: (alert) => {
+      if (isAlert(alert)) {
+        addAlert(alert);
+      }
+    },
+  });
+
+  const seededRef = useRef(false);
+
+  useEffect(() => {
+    if (!ready || seededRef.current) {
+      return;
+    }
+
+    const createEvent = postEvent('CREATE_EVENT');
+    createEvent({
+      type: 'earthquake',
+      latitude: 37.7749,
+      longitude: -122.4194,
+      name: 'Training Scenario',
+    });
+
+    const addZone = postEvent('ADD_ZONE');
+    addZone({
+      name: 'Zone A',
+      zone_type: 'rectangle',
+      x: 60,
+      y: 60,
+      width: 180,
+      height: 120,
+    });
+    addZone({
+      name: 'Zone B',
+      zone_type: 'circle',
+      center_x: 300,
+      center_y: 170,
+      radius: 60,
+    });
+
+    upsertEvent({
+      event_id: 'training-scenario',
+      disaster_type: 1,
+      latitude: 37.7749,
+      longitude: -122.4194,
+      description: 'Training Scenario',
+    });
+
+    seededRef.current = true;
+  }, [postEvent, upsertEvent, ready]);
+
+  useEffect(() => {
+    if (ready && lastFrame) {
+      sendFrameUpdate(lastFrame);
+    }
+  }, [lastFrame, ready, sendFrameUpdate]);
+
+  const { height } = useWindowDimensions();
+  const webHeight = Math.max(240, Math.floor(height * 0.5));
+
+  return (
+    <ThemedView style={{ flex: 1, backgroundColor: colors.bg, padding: spacing.md }}>
+      <ConnectionBanner status={resolveBannerState(connectionStatus)} />
+      <View style={{ marginTop: 20 }}>
+        <SurvivorCounter survivors={survivors} />
+      </View>
+      <View style={{ height: webHeight }}>
+        <MatWebView
+          webViewRef={webViewRef}
+          onMessage={onMessage}
+          style={{ flex: 1, borderRadius: 12, overflow: 'hidden', backgroundColor: colors.surface }}
+        />
+      </View>
+      <View style={{ flex: 1, marginTop: spacing.md }}>
+        <AlertList alerts={alerts} />
+      </View>
+    </ThemedView>
+  );
+};
+
+export default MATScreen;