MerginProject may not get cleaned up properly

When MerginProject is created, it creates geodiff object and assigns an internal logger to geodiff. This creates a cyclic reference (pygeodiff refs MerginProject, and MerginProject refs pygeodiff) that may keep resources held, which may be a problem if MerginProject object is created many times. See also fix for this in db-sync: https://github.com/MerginMaps/mergin-db-sync/pull/79

Also somehow related: https://github.com/MerginMaps/geodiff/issues/190

Code snippet to replicate the cleanup issue:

import pygeodiff

class MerginProjectX:

    def __init__(self, directory):

        def _logger_callback(level, text_bytes):
            self.x()   # not a real function, but refs this object...

        self.geodiff = pygeodiff.GeoDiff()
        self.geodiff.set_logger_callback(_logger_callback)

# this will keep consuming more and more resources
for i in range(50000):
    print(i)
    mp = MerginProjectX("x")
    #mp.geodiff = None    # this helps to break ref cycle
    time.sleep(0.005)

首先定位 MerginProject 的 pygeodiff GeoDiff 建立以及 logger-callback 設定，然後比較 mergin-db-sync PR 79 中的清理修正與相關的 geodiff issue 190。完成標準是：重複建立 MerginProject 不再保留循環參照，也不會導致資源增長，並使用重現迴圈驗證清理效果。