VRT writer maps 64-bit integer rasters to wrong dtype

[brendancol]

Describe the bug

`write_vrt` in `xrspatial/geotiff/_vrt.py` emits the wrong `dataType` for 64-bit integer source rasters. The dtype lookup at line 1366-1368 has no entry for `bps=64`:

```python
elif sf == 2:
vrt_dtype_name = {8: 'Int8', 16: 'Int16', 32: 'Int32'}.get(bps, 'Int32')
else:
vrt_dtype_name = {8: 'Byte', 16: 'UInt16', 32: 'UInt32'}.get(bps, 'Byte')
```

Signed 64-bit falls back to `Int32` and unsigned 64-bit falls back to `Byte`. The VRT reader has explicit `Int64` and `UInt64` support (`_vrt.py:183-184`), so the writer is the only side that drops the precision.

Reproduce

Write a uint64 raster via `to_geotiff(da, "out.vrt")`. The emitted VRT declares `dataType="Byte"` and reading it back returns uint8 with values truncated to 255.

Expected behavior

64-bit integer rasters should round-trip through VRT without dtype loss. Signed 64-bit should emit `Int64` and unsigned 64-bit should emit `UInt64`.

Fix

Add `64: 'Int64'` to the signed branch and `64: 'UInt64'` to the unsigned branch of the lookup.