feat: simplify Show4DSTEM dataset comparison

[bobleesj]

What problem does it solve?

Opening one 4D-STEM master and comparing several compatible masters required users to understand internal layout parameters. The common workflow is now the same in both cases: load(...), then Show4DSTEM(data). A loaded 5D stack opens in Multiple view with filename labels and Selected diffraction mode automatically.

What should reviewer(s) do?

Review the new one-dataset and multi-dataset guides. Confirm that Show4DSTEM(load([master1, master2])) opens the comparison view without extra layout parameters, then switch Selected to Average and back to Selected.

Verification

• npm run build
• 31 passed in tests/show4dstem/test_factory.py
• Full clean-tree suite: 1118 passed, 20 skipped, 1 unrelated ShowPtycho failure caused by the external local quantem.gpu checkout mismatch
• jupyter-book build docs
• Documentation navigation, page-size, and notebook-size guards passed
• Manual Phil WebGPU multi-tilt signoff: progressive panels, Selected, Average, and detector interaction verified at native detector sampling

---

This PR follows the scikit-package workflow for reproducible scientific software.