Fix GitHub notebook widget publishing and 4D-STEM parity

[bobleesj]

What problem does it solve?

GitHub-ready notebooks could render widgets twice or capture folder widgets
before their first complete frame. This keeps one complete UI per cell and
includes the validated Show4DSTEM GPU-parity fixes.

What should reviewer(s) do?

Review the notebook publisher, folder readiness, and Show4DSTEM parity tests.

Verification

• Python: 1175 passed; frontend: 172 passed.
• Eleven CUDA drift notebooks passed end to end.
• 28 widget cells: 28 complete UIs, zero duplicates or saved errors.

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

[github-actions]

• Delete bobleesj/quantem.widget:fix/github-full-widget-output from its remote if you own it, it is not long-lived, and no other PR uses it.
• Remove the dedicated worktree and local fix/github-full-widget-output branch after confirming the worktree is clean.
• Fetch electronmicroscopy/quantem.widget and update local main before starting new work.

[github-actions]

This PR is 1 commit behind electronmicroscopy/quantem.widget:main.

Fetch that remote and update local main before you branch, or rebase this branch onto it before review.
A warning is enough. The PR can stay open while main moves.