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/ZonesScreen/useOccupancyGrid.ts

@@ -0,0 +1,109 @@
+import { useMemo } from 'react';
+import type { Classification, SignalField } from '@/types/sensing';
+import { usePoseStore } from '@/stores/poseStore';
+
+const GRID_SIZE = 20;
+const CELL_COUNT = GRID_SIZE * GRID_SIZE;
+
+type Point = {
+  x: number;
+  y: number;
+};
+
+const clamp01 = (value: number): number => {
+  if (Number.isNaN(value)) {
+    return 0;
+  }
+
+  return Math.max(0, Math.min(1, value));
+};
+
+const parseNumber = (value: unknown): number | null => {
+  return typeof value === 'number' && Number.isFinite(value) ? value : null;
+};
+
+const parsePoint = (value: unknown): Point | null => {
+  if (!value || typeof value !== 'object') {
+    return null;
+  }
+
+  const record = value as Record<string, unknown>;
+  const x = parseNumber(record.x);
+  const y = parseNumber(record.y);
+
+  if (x === null || y === null) {
+    return null;
+  }
+
+  return {
+    x,
+    y,
+  };
+};
+
+const collectPositions = (value: unknown): Point[] => {
+  if (!Array.isArray(value)) {
+    return [];
+  }
+
+  return value
+    .map((entry) => parsePoint(entry))
+    .filter((point): point is Point => point !== null)
+    .map((point) => ({
+      x: point.x,
+      y: point.y,
+    }));
+};
+
+const readClassificationPositions = (classification: Classification | undefined): Point[] => {
+  const source = classification as unknown as Record<string, unknown>;
+
+  return (
+    collectPositions(source?.persons) ??
+    collectPositions(source?.personPositions) ??
+    collectPositions(source?.positions) ??
+    []
+  );
+};
+
+export const useOccupancyGrid = (signalField: SignalField | null): { gridValues: number[]; personPositions: Point[] } => {
+  const classification = usePoseStore((state) => state.classification) as Classification | undefined;
+
+  const gridValues = useMemo(() => {
+    const sourceValues = signalField?.values;
+
+    if (!sourceValues || sourceValues.length === 0) {
+      return new Array(CELL_COUNT).fill(0);
+    }
+
+    const normalized = new Array(CELL_COUNT).fill(0);
+    const sourceLength = Math.min(CELL_COUNT, sourceValues.length);
+
+    for (let i = 0; i < sourceLength; i += 1) {
+      const value = parseNumber(sourceValues[i]);
+      normalized[i] = clamp01(value ?? 0);
+    }
+
+    return normalized;
+  }, [signalField?.values]);
+
+  const personPositions = useMemo(() => {
+    const positions = readClassificationPositions(classification);
+
+    if (positions.length > 0) {
+      return positions
+        .map(({ x, y }) => ({
+          x: Math.max(0, Math.min(GRID_SIZE - 1, x)),
+          y: Math.max(0, Math.min(GRID_SIZE - 1, y)),
+        }))
+        .slice(0, 16);
+    }
+
+    return [] as Point[];
+  }, [classification]);
+
+  return {
+    gridValues,
+    personPositions,
+  };
+};