feat: access more frames via direct read_frame() method (#192)

* access more frames

* style(pre-commit.ci): auto fixes [...]

* fix lint

* remove

* remove dir

* no walrus

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/nd2/readers/_modern/modern_reader.py

@@ -310,12 +310,16 @@ def _frame_offsets(self) -> dict[int, int]:
 
     def read_frame(self, index: int) -> np.ndarray:
         """Read a chunk directly without using SDK."""
-        if index > self._seq_count():
-            raise IndexError(f"Frame out of range: {index}")
         if not self._fh:  # pragma: no cover
             raise ValueError("Attempt to read from closed nd2 file")
+
+        # sometimes a frame index has a valid offset even if it's greater than
+        # _seq_count() (for example, if experiment parsing misses stuff)
+        # so, it should still be accessible.
         offset = self._frame_offsets.get(index, None)
         if offset is None:
+            if index > self._seq_count():  # pragma: no cover
+                raise IndexError(f"Frame out of range: {index}")
             return self._missing_frame(index)
 
         if self.attributes().compressionType == "lossless":