feat(search): coordinates and bundled presets resolve keyless (#572)

Supersedes #388 by @KuraPiee, ported onto the current component layout with fixes from review.

Verified before merge: format, package boundaries, unit suite, build, and the browser tracking suite; a browser harness for the change where one applies; adversarial review applied.

Co-authored-by: KuraPiee <[email protected]>

src/search/coordinateGeocoder.js

@@ -0,0 +1,57 @@
+import { parseCoordinateQuery } from './coordinateParser.js';
+
+/** Half-width of the box a bare coordinate is framed with, in degrees. */
+const COORDINATE_HALF_SPAN_DEG = 0.015;
+
+/** Keep a generated box inside the poles and inside the dateline. */
+function boundedBox(lat, lon, halfSpan) {
+  const wrap = (value) => {
+    const wrapped = ((((value + 180) % 360) + 360) % 360) - 180;
+    return wrapped === -180 ? 180 : wrapped;
+  };
+  return {
+    southwest: {
+      lat: Math.max(-90, lat - halfSpan),
+      lng: wrap(lon - halfSpan),
+    },
+    northeast: {
+      lat: Math.min(90, lat + halfSpan),
+      lng: wrap(lon + halfSpan),
+    },
+  };
+}
+
+/**
+ * Answer a decimal-degree query without asking anyone.
+ *
+ * It sits ahead of the network geocoders, so a coordinate costs no request and
+ * works with no key. Anything that is not exactly a coordinate is passed on
+ * untouched, and `answered: true` on a decline means only that this provider
+ * had nothing to say — it is not a verdict on the query.
+ */
+export function createCoordinateGeocoder() {
+  return {
+    async geocode(query, { signal } = {}) {
+      signal?.throwIfAborted();
+      const parsed = parseCoordinateQuery(query);
+      if (!parsed) return { place: null, answered: true };
+      return {
+        place: {
+          lat: parsed.lat,
+          lng: parsed.lon,
+          name: parsed.label,
+          label: parsed.label,
+          types: ['coordinate'],
+          // The point is the answer; nothing nearby can improve on it.
+          exact: true,
+          viewport: boundedBox(
+            parsed.lat,
+            parsed.lon,
+            COORDINATE_HALF_SPAN_DEG,
+          ),
+        },
+        answered: true,
+      };
+    },
+  };
+}