feat(block2): geohash encoder in commons_core (for 'use my location')

Pure Dart lat/lon -> low-precision geohash (public-domain algorithm), the piece
the coarse-area capture needs so people never type a code. Precision-controlled
(5 chars ~ ±2.4 km). Tested against the canonical u4pruydqqvj vector + prefix/
proximity properties. 5 tests green.

packages/commons_core/test/social/geohash_test.dart

@@ -0,0 +1,35 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Geohash.encode', () {
+    test('matches the classic reference vector', () {
+      // (57.64911, 10.40744) → "u4pruydqqvj" is the canonical geohash example.
+      expect(Geohash.encode(57.64911, 10.40744, precision: 11),
+          'u4pruydqqvj');
+    });
+
+    test('precision controls length and coarseness (prefix relation)', () {
+      final fine = Geohash.encode(57.64911, 10.40744, precision: 9);
+      final coarse = Geohash.encode(57.64911, 10.40744, precision: 5);
+      expect(coarse.length, 5);
+      expect(fine, startsWith(coarse)); // coarser is a prefix of finer
+    });
+
+    test('nearby points share a coarse prefix; far ones do not', () {
+      final a = Geohash.encode(41.39, 2.16, precision: 5); // Barcelona-ish
+      final b = Geohash.encode(41.40, 2.17, precision: 5); // ~1.5 km away
+      final far = Geohash.encode(40.41, -3.70, precision: 5); // Madrid
+      expect(a.substring(0, 3), b.substring(0, 3));
+      expect(a, isNot(far));
+    });
+
+    test('origin encodes without error', () {
+      expect(Geohash.encode(0, 0, precision: 5), hasLength(5));
+    });
+
+    test('rejects non-positive precision', () {
+      expect(() => Geohash.encode(0, 0, precision: 0), throwsArgumentError);
+    });
+  });
+}