fix(dart): use BigInt for proper dart web truncation

.github/workflows/ci.yml

@@ -999,6 +999,10 @@ jobs:
         run: |
           cd dart/packages/fory-test
           dart test
+      - name: Run tests (Chrome)
+        run: |
+          cd dart/packages/fory
+          dart test -p chrome
       - name: Run code analysis
         run: |
           cd dart/packages/fory-test

dart/packages/fory/lib/src/buffer.dart

@@ -370,7 +370,7 @@ final class Buffer {
       return;
     }
     final signed = BigInt.from(value);
-    final zigZag = ((signed << 1) ^ BigInt.from(value >> 63)) & _mask64Big;
+    final zigZag = ((signed << 1) ^ (signed >> 63)) & _mask64Big;
     _writeVarUint64BigInt(zigZag);
   }
 

dart/packages/fory/lib/src/codegen/generated_support.dart

@@ -178,7 +178,7 @@ final class GeneratedWriteCursor {
     }
     final signed = BigInt.from(value);
     final zigZag =
-        ((signed << 1) ^ BigInt.from(value >> 63)) & _generatedCursorMask64Big;
+        ((signed << 1) ^ (signed >> 63)) & _generatedCursorMask64Big;
     _writeVarUint64BigInt(zigZag);
   }
 

dart/packages/fory/test/buffer_test.dart

@@ -263,6 +263,31 @@ void main() {
       }
     });
 
+    test('varint64 round-trips positive values >= 2^31 (web zig-zag regression)', () {
+        const cases = <int>[
+          2147483648, // 2^31 — first value that triggers 32-bit sign confusion
+          3000000000,
+          4294967295, // 2^32 - 1
+          0x200000000, // 2^33
+          1 << 34,
+          1 << 42,
+          1 << 50,
+          0x7fffffffffffffff, // i64::MAX
+        ];
+        for (final value in cases) {
+          final buffer = Buffer();
+          buffer.writeVarInt64(value);
+          final readBuffer = Buffer.wrap(buffer.toBytes());
+          final decoded = readBuffer.readVarInt64();
+          expect(
+            decoded,
+            equals(value),
+            reason: 'writeVarInt64($value) round-tripped to $decoded',
+          );
+        }
+      },
+    );
+
     test('round-trips tagged int64 boundary values with Java-aligned lengths',
         () {
       const cases = <({int bytes, int value})>[