fix: keep the mirror readable under a umask that masks the owner's read bit

The umask handling added for group access cleared only the group bits and left
owner and other to the environment. A container whose umask carries 0400 then
produces mirror directories of mode 0300: writable and enterable, unreadable to
the very run that created them, and unreadable to anything serving the share.

Clear the owner read and execute bits from the umask as well. The `other` bits
stay where the environment puts them, because whether the mirror is
world-readable is a real policy question; being able to read a directory the
process itself just created is not.

Files were never exposed to this: mkstemp sets 0600 outright and copy2 takes
the source file's mode, both ignoring the umask.

tests/conftest.py

@@ -27,6 +27,21 @@ def tight_umask():
     os.umask(previous)
 
 
+@pytest.fixture
+def owner_hostile_umask():
+    """Return a callable applying a umask that masks off the owner's read bit.
+
+    Unusual, but it is what produces a mirror tree of mode 0300 -- writable and
+    enterable, unreadable to the very process that built it. Applied on demand
+    rather than for the whole test, because the source library is built by
+    something else entirely and the same umask would make the test's own
+    fixtures unreadable before the run under test even started.
+    """
+    previous = os.umask(0o022)
+    yield lambda: os.umask(0o477)
+    os.umask(previous)
+
+
 @pytest.fixture
 def make_flac():
     """Return a factory writing a short tagged FLAC file."""