feat(transit): add Szczecin buses and trams (ZDiTM, CC0)

Keyless GTFS-Realtime feed published by ZDiTM Szczecin under CC0 1.0.
Speeds are m/s (checked against vehicle movement), the file refreshes about
every 20 s, and the feed carries no route ids, so vehicles take the default
bus mode.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

src/data/transitFeeds.js

@@ -788,6 +788,29 @@ export const TRANSIT_FEED_REGISTRY = Object.freeze([
     defaultMode: 'bus',
     routeLabel: rioRouteLabel,
   }),
+  Object.freeze({
+    id: 'zditm-szczecin',
+    name: 'Szczecin buses and trams',
+    operator:
+      'Zarząd Dróg i Transportu Miejskiego w Szczecinie (ZDiTM), Szczecin',
+    region: 'Szczecin, Poland',
+    // Every vehicle in a 2026-11-05 snapshot sat between 53.34–53.55 N and
+    // 14.44–14.77 E, inside 20 km of this point.
+    center: Object.freeze({ lat: 53.44, lon: 14.56 }),
+    loadRadiusKm: 25,
+    url: 'https://www.zditm.szczecin.pl/storage/gtfs/gtfs-rt-vehicles.pb',
+    license: 'CC0 1.0',
+    licenseUrl: 'https://www.zditm.szczecin.pl/en/zditm/for-developers',
+    attribution:
+      'Zarząd Dróg i Transportu Miejskiego w Szczecinie (ZDiTM Szczecin)',
+    defaultEnabled: true,
+    terms: Object.freeze({
+      quote:
+        'The data are provided free of charge under the CC0 1.0 licence, for use in any context, with no prior notification required.',
+      note: 'CC0 needs no credit; it is given anyway as a courtesy. ZDiTM gives no guarantee of accuracy or completeness. The developer page says the feed updates every minute; measured on 2026-11-05 the file changed about every 20 seconds, with vehicle timestamps up to 50 seconds old. Speeds are m/s: checked against vehicle movement between two snapshots. The feed carries no route id at all, so every vehicle takes the feed default mode even though the city also runs trams.',
+    }),
+    defaultMode: 'bus',
+  }),
 ]);
 
 /**

src/data/transitFeeds.test.mjs

@@ -552,3 +552,29 @@ test('Rio feeds are CC BY 4.0 and live', () => {
     'Rio has a DATA_SOURCES row',
   );
 });
+
+test('Szczecin is CC0, live, covers the city and not Berlin', () => {
+  const feed = getTransitFeed('zditm-szczecin');
+  assert.equal(feed.defaultEnabled, true);
+  assert.equal(feed.license, 'CC0 1.0');
+  assert.equal(
+    feed.licenseUrl,
+    'https://www.zditm.szczecin.pl/en/zditm/for-developers',
+  );
+  assert.match(feed.terms.quote, /CC0 1\.0 licence/);
+  assert.match(feed.attribution, /ZDiTM/);
+  assert.equal(feed.routeLabel, undefined, 'the feed has no route ids');
+  assert.equal(transitModeFor(feed, null), 'bus');
+  const ids = (lat, lon) => transitFeedsInRange(lat, lon).map((f) => f.id);
+  assert.ok(ids(53.4285, 14.5528).includes('zditm-szczecin'), 'city centre');
+  assert.ok(ids(53.5, 14.7).includes('zditm-szczecin'), 'north-east edge');
+  assert.ok(!ids(52.52, 13.405).includes('zditm-szczecin'), 'Berlin');
+  const sources = readFileSync(
+    new URL('../../DATA_SOURCES.md', import.meta.url),
+    'utf8',
+  );
+  assert.ok(
+    sources.split('\n').some((l) => l.startsWith('| **ZDiTM Szczecin')),
+    'Szczecin has a DATA_SOURCES row',
+  );
+});