feat: describe what the app shows as a view

A view is a camera, data layers, visual style, map imagery and an aircraft,
military aircraft or satellite to follow, written in the share-link format
the app already restores. gods-eye-view/view builds, encodes and decodes one,
and share links take their style names from it. open_in_gods_eye_view now
builds its link from a full view: an area or camera, layers, style, map and
something to follow.

src/view/index.test.mjs

@@ -0,0 +1,79 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  STYLE_URL_NAMES,
+  createView,
+  viewFromParams,
+  viewToParams,
+  viewUrl,
+} from './index.js';
+
+test('a view round-trips through share-link parameters', () => {
+  const view = createView({
+    camera: {
+      lat: 35.68,
+      lon: 139.76,
+      altitude_m: 300000,
+      heading_deg: 15,
+      pitch_deg: -45,
+    },
+    layers: ['earthquakes', 'ais-live-vessels'],
+    style: 'retro',
+    map: 'osm',
+    follow: { kind: 'aircraft', id: 'AE1234' },
+  });
+  const params = viewToParams(view);
+  assert.equal(params.get('style'), 'crt');
+  assert.equal(params.get('map'), 'osm');
+  assert.deepEqual(viewFromParams(params), view);
+  assert.deepEqual(view.layers, ['ais-live-vessels', 'earthquakes', 'flights']);
+  const satellite = createView({
+    camera: { lat: 0, lon: 0 },
+    follow: { kind: 'satellite', id: '25544' },
+  });
+  assert.deepEqual(viewFromParams(viewToParams(satellite)).follow, {
+    kind: 'satellite',
+    id: '25544',
+  });
+});
+
+test('views have defaults, bounds, and reject what links cannot carry', () => {
+  const view = createView({ camera: { lat: 91, lon: 10, altitude_m: 1 } });
+  assert.deepEqual(view.camera, {
+    lat: 90,
+    lon: 10,
+    altitude_m: 50,
+    heading_deg: 0,
+    pitch_deg: -90,
+  });
+  assert.equal(view.style, null);
+  const params = viewToParams(view);
+  assert.equal(params.has('style'), false);
+  assert.equal(params.has('l'), false);
+  assert.throws(() => createView({ camera: {} }), /lat and lon/);
+  assert.throws(
+    () => createView({ camera: { lat: 0, lon: 0 }, style: 'sepia' }),
+    /Unknown view style/,
+  );
+  assert.throws(
+    () =>
+      createView({
+        camera: { lat: 0, lon: 0 },
+        follow: { kind: 'ship', id: '1' },
+      }),
+    /Cannot follow a ship/,
+  );
+  assert.equal(viewFromParams(new URLSearchParams('style=crt')), null);
+  assert.equal(
+    new URL(viewUrl('https://maps.example/app', view)).hash.slice(1),
+    params.toString(),
+  );
+});
+
+test('views use the same style names as the app share links', async () => {
+  const source = await import('node:fs/promises').then((fs) =>
+    fs.readFile(new URL('../sharelink.js', import.meta.url), 'utf8'),
+  );
+  assert.match(source, /const STYLE_TO_URL = STYLE_URL_NAMES;/);
+  assert.equal(STYLE_URL_NAMES.retro, 'crt');
+});