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/lib/src/social/geohash.dart

@@ -0,0 +1,56 @@
+/// Geohash encoding (public-domain algorithm), pure Dart. Turns a latitude /
+/// longitude into a short base-32 string whose length sets its precision — the
+/// coarse "near you" area used by offers. Kept deliberately low-precision at the
+/// call site so an exact position never leaves the device.
+class Geohash {
+  const Geohash._();
+
+  static const _base32 = '0123456789bcdefghjkmnpqrstuvwxyz';
+
+  /// Encodes [latitude]/[longitude] to a geohash of [precision] characters.
+  /// Precision ≈ cell size: 5 ≈ ±2.4 km, 4 ≈ ±20 km, 3 ≈ ±78 km.
+  static String encode(
+    double latitude,
+    double longitude, {
+    int precision = 5,
+  }) {
+    if (precision < 1) throw ArgumentError.value(precision, 'precision');
+    var latMin = -90.0, latMax = 90.0;
+    var lonMin = -180.0, lonMax = 180.0;
+    final out = StringBuffer();
+    var bit = 0;
+    var ch = 0;
+    var even = true; // start with longitude
+
+    while (out.length < precision) {
+      if (even) {
+        final mid = (lonMin + lonMax) / 2;
+        if (longitude >= mid) {
+          ch = (ch << 1) | 1;
+          lonMin = mid;
+        } else {
+          ch = ch << 1;
+          lonMax = mid;
+        }
+      } else {
+        final mid = (latMin + latMax) / 2;
+        if (latitude >= mid) {
+          ch = (ch << 1) | 1;
+          latMin = mid;
+        } else {
+          ch = ch << 1;
+          latMax = mid;
+        }
+      }
+      even = !even;
+      if (bit < 4) {
+        bit++;
+      } else {
+        out.write(_base32[ch]);
+        bit = 0;
+        ch = 0;
+      }
+    }
+    return out.toString();
+  }
+}