chore: delete legacy flat data paths from HF quantem-data after grace period

[bobleesj]

The tutorial data on https://huggingface.co/datasets/bobleesj/quantem-data now lives under the widget-organized tree widget-tutorials/<widget>/<dataset>/<size>/. All consumers were migrated on 2026-07-10 (loaders in f701228, both Colab gists, the notebooks/ mirrors, and the dataset card).

The old flat paths are kept temporarily so notebooks that are mid-workshop and the released rc27 loaders keep working. Delete them once BOTH are true:

1. An rc containing f701228 (widget-tutorials-only loaders) is the current TestPyPI release.
2. Roughly two weeks have passed and the Colab gists have been re-verified on the new paths (/colab-drive).

Paths to delete (plus their .thumbnails/ entries where present):

• 4dstem/gold_512_npy_bin8/ (mirrored to widget-tutorials/show4dstem/gold-512-bin8/full/)
• 4dstem/gold_512_npy_bin4/ (mirrored to widget-tutorials/show4dstem/gold-512-bin4/full/)
• 4dstem/gold_128_npy_bin8/ (mirrored to widget-tutorials/show4dstem/gold-128-bin8/full/)
• haadf/gold_haadf_npy/ (mirrored to widget-tutorials/shared/gold-haadf/full/)
• survey/gold_haadf_session/ (mirrored to widget-tutorials/showfolder/gold-haadf-session/small/)

Keep: 4dstem/gold_512/, 4dstem/gold_30mrad1.3mx*/, haadf/*.emd — those are the full-size power-user originals, not tutorial bundles.

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