CVE-2026-96639

### Summary Pillow multi-frame image loaders can retain native `ImagingCore` storage allocated for one frame while changing the Python-side image `mode` and decoder tiles for a later frame. If frame 0 allocates storage for a narrow mode such as `L`, and a later frame of the same dimensions switches to a wider mode such as `RGB` or `CMYK`, Pillow may decode wider pixels into the stale narrow native buffer. This causes native out-of-bounds writes and process crashes. In the MPO/JPEG path, I also demonstrated command execution in matching Python/Pillow runtime profiles through Pillow's documented `ImageSequence.Iterator` + `frame.load()` path. ### Details The root cause appears to be that `ImageFile.load_prepare()` only reallocates native storage when `_im is None`. It does not verify that the retained native `ImagingCore` storage still matches `self.mode` and `self.size`. Several multi-frame `seek()` / `_open()` paths can update `self._mode` and decoder tiles for a later frame without invalidating `_im`. The decoder then writes or reads according to the new frame mode while the native storage still belongs to the old frame mode. Verified affected surfaces: 1. **PSD** `PsdImageFile.seek()` can switch from grayscale/native `L` storage to RGB layer decoding while retaining `_im`. This produces attacker-controlled native out-of-bounds writes and reliable crashes. 2. **MPO/JPEG** A crafted two-frame MPO can make frame 0 allocate `L` storage, then make frame 1 decode `CMYK` / `RGB` data into the retained `L` storage. The documented `ImageSequence.Iterator` path reaches this with `frame.load()`. Verified native memory corruption, crash, over-read behavior, and scoped harmless-marker command execution on Pillow 12.3.0. 3. **DCX** `DcxImageFile.seek()` calls `PcxImageFile._open()`, which changes mode/tile state without a native-storage mode guard. A crafted DCX with `L` then `RGB` PCX frames produces retained-storage mismatch and crash. 4. **MIC** M