feat: scaffold Expo Prebuild project with all v1 screens and services

Sets up the full bincio-rec source tree: Zustand recording store with
haversine stats, background GPS via expo-task-manager, BLE scan/subscribe
for HR and power, GPX writer with Garmin extensions, SQLite recordings
list, multipart upload to bincio-activity, React Navigation stack with
bottom tabs, and build instructions in README.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/services/gpx.ts

@@ -0,0 +1,64 @@
+import { File, Directory, Paths } from 'expo-file-system';
+import { TrackPoint } from '../types';
+
+function recordingsDir(): Directory {
+  return new Directory(Paths.document, 'recordings');
+}
+
+export function ensureRecordingsDir(): void {
+  const dir = recordingsDir();
+  if (!dir.exists) dir.create();
+}
+
+export function saveGpx(trackPoints: TrackPoint[], title: string): string {
+  ensureRecordingsDir();
+  const filename = `${sanitizeFilename(title)}_${Date.now()}.gpx`;
+  const file = new File(recordingsDir(), filename);
+  file.write(buildGpx(trackPoints, title));
+  return file.uri;
+}
+
+export function buildGpx(trackPoints: TrackPoint[], title: string): string {
+  const trkpts = trackPoints.map(buildTrkpt).join('\n');
+  return `<?xml version="1.0" encoding="UTF-8"?>
+<gpx version="1.1" creator="bincio-rec"
+  xmlns="http://www.topografix.com/GPX/1/1"
+  xmlns:gpxtpx="http://www.garmin.com/xmlschemas/TrackPointExtension/v1">
+  <metadata><name>${escapeXml(title)}</name></metadata>
+  <trk>
+    <name>${escapeXml(title)}</name>
+    <trkseg>
+${trkpts}
+    </trkseg>
+  </trk>
+</gpx>`;
+}
+
+function buildTrkpt(pt: TrackPoint): string {
+  const ext = buildExtensions(pt);
+  return `      <trkpt lat="${pt.lat}" lon="${pt.lon}">
+        <ele>${pt.ele.toFixed(1)}</ele>
+        <time>${pt.time.toISOString()}</time>${ext}
+      </trkpt>`;
+}
+
+function buildExtensions(pt: TrackPoint): string {
+  if (pt.hr == null && pt.power == null && pt.cad == null) return '';
+  const hr = pt.hr != null ? `<gpxtpx:hr>${pt.hr}</gpxtpx:hr>` : '';
+  const power = pt.power != null ? `<gpxtpx:power>${pt.power}</gpxtpx:power>` : '';
+  const cad = pt.cad != null ? `<gpxtpx:cad>${pt.cad}</gpxtpx:cad>` : '';
+  return `
+        <extensions>
+          <gpxtpx:TrackPointExtension>
+            ${hr}${power}${cad}
+          </gpxtpx:TrackPointExtension>
+        </extensions>`;
+}
+
+function escapeXml(s: string): string {
+  return s.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;');
+}
+
+function sanitizeFilename(s: string): string {
+  return s.replace(/[^a-zA-Z0-9_-]/g, '_').slice(0, 64);
+}