Add interactive region annotation tutorial (skeleton)

[timtreis]

Summary

Adds a new gallery entry at examples/interactive_annotate.ipynb covering the sdata.pl.annotate() API introduced in scverse/spatialdata-plot#684. Uses the same squidpy.datasets.visium_hne_sdata() dataset as visium_mouse_brain.ipynb so the cache is shared.

Structure follows the existing examples/ pattern (intro → load → progressive feature demos → reproducibility):

• Intro markdown with dataset citation, anywidget link, and the pip install 'spatialdata-plot[interactive]' hint
• Load + initial render_images(\"hne\") so the reader sees what they're outlining
• Widget invocation as a markdown fenced block, not a code cell — the static docs build can't execute the anywidget JS runtime, so the snippet is illustrative; live-kernel users copy and run it
• GIF placeholder cell (interactive_annotate.gif) for the visual demo in the rendered docs
• A "simulated commit" code cell that builds the equivalent ShapesModel directly via shapely + ShapesModel.parse(transformations={cs: Identity()}), so every downstream cell is reproducible in the docs build without the widget
• Overlay with render_shapes; polygon-crop via sd.polygon_query
• Watermark block matching color_and_palette.ipynb

examples/index.md updated with the new gallery card and toctree entry.

Outstanding before this can go from draft to ready

• Replace the hippocampus polygon coordinates in the simulated-commit cell with values from a real annotation pass — they're currently a placeholder
• Record interactive_annotate.gif (~3-5 seconds of drawing + saving)
• Add interactive_annotate.png thumbnail for the gallery grid card
• Re-execute the notebook end-to-end (Restart Kernel & Run All) and commit outputs, per the contributing guide

Test plan

• PR build preview renders the new card on the examples index
• Live notebook run shows the widget and the simulated-commit cell produces an overlay matching the GIF capture
• CI execute.yaml re-executes without hanging on the widget cell (it shouldn't — the widget invocation is markdown, not code)

Depends on

• Add sdata.pl.annotate() — interactive region selection via anywidget spatialdata-plot#684 (the sdata.pl.annotate() API itself)

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