Handle special filesystem entries explicitly (#11)

* Handle special filesystem entries explicitly

- Added fs.entry_kind(): classifies via lstat() mode bits alone
  ("file", "dir", "symlink", "fifo", "socket", "block_device",
  "char_device", "unknown") - never opens the entry, so classifying a
  FIFO can't block even without a writer.
- rsync() now classifies every source entry before touching it and
  yields a new Actions.EXCLUDE for anything outside file/dir/symlink,
  instead of falling through to copy_direntry()'s file branch, which
  would either block forever opening a writerless FIFO for read, or
  raise an OSError that initiate_backup() treats as fatal (aborting the
  whole backup for one socket).
- initiate_backup() records exclusions in the completion manifest
  (keyed by snapshot-relative path, valued with the excluded kind), and
  no longer nest_hardlinks or counts a recurring exclusion as a change
  worth keeping a new snapshot for.
- Chose the "recorded exclusion" policy the plan allows, rather than
  fail-fast: a FIFO or socket living in a backed-up tree is not itself
  a backup failure, and previously turned into one.

Note: rsync_ext() (external rsync) is unaffected by this change and
keeps whatever its own --archive flags do with special files; the P1
item on Python/external implementation parity covers reconciling that.
The pre-existing test_src_is_socket failure (AF_UNIX path length on
macOS tmp_path) is untouched here - it's separately tracked under P1
test fixes - but this PR adds working socket/FIFO/char-device coverage
via a short-path tempdir.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Address Copilot review: fix stale exclude, exclusion-only diffs, test portability

- fs.rsync(): remove the existing dst_entry before yielding EXCLUDE for
  a src entry with an unsupported kind, so an entry inherited via
  hardlink from a prior snapshot doesn't linger in the new one.
- backup.initiate_backup(): compare this run's excluded set against
  the previous snapshot's manifest; a change in exclusions alone now
  counts as backup_changed, so the updated manifest gets written.
- tests: skip mkfifo/AF_UNIX-dependent tests on platforms without
  support, and use TemporaryDirectory/context managers instead of
  manual mkdtemp/unlink for more robust cleanup.

* Remove unused shutil import from test_rsync.py

Left over after switching the socket/fifo tests to
tempfile.TemporaryDirectory(), per Copilot's suppressed comment.

* Fix rm_direntry() no-op on special dst entries

rm_direntry() only handled file/symlink/dir - a FIFO, socket, or
device node at dst (e.g. inherited from an external-rsync snapshot)
silently survived removal, since is_file/is_dir/is_symlink all
return False for it. Any non-directory entry is unlinkable the same
way, so invert the branching: recurse for directories, unlink
everything else.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_integration.py

@@ -1081,3 +1081,35 @@ def test_external_rsync_snapshot_still_gets_checksums(integration_dirs):
     manifest = bk._read_manifest(bk._get_backup_marker(snapshot).path)
     assert manifest["checksums"]
     assert bk.verify_snapshot(snapshot) == []
+
+
+@pytest.mark.skipif(not hasattr(os, "mkfifo"),
+                    reason="os.mkfifo not available on this platform")
+def test_fifo_excluded_and_recorded_without_blocking(integration_dirs):
+    """
+    A FIFO in the source must not block the backup (no writer is ever
+    attached), must not abort it, and must be recorded in the manifest
+    as an exclusion rather than silently disappearing.
+    """
+    backups_dir, source_dir = integration_dirs
+    (source_dir / "file1.txt").write_text("content1")
+    os.mkfifo(str(source_dir / "a_fifo"))
+
+    snapshot = bk.initiate_backup(
+        sources=[str(source_dir)],
+        backups_dir=str(backups_dir),
+        dry_run=False,
+    )
+
+    assert snapshot is not None
+    source_name = os.path.basename(str(source_dir))
+    manifest = bk._read_manifest(bk._get_backup_marker(snapshot).path)
+    assert manifest["excluded"] == {
+        os.path.join(source_name, "a_fifo"): "fifo"
+    }
+    assert not os.path.lexists(
+        os.path.join(snapshot.path, source_name, "a_fifo")
+    )
+    assert os.path.exists(
+        os.path.join(snapshot.path, source_name, "file1.txt")
+    )