Make Atlas experimental search results selectable and batch-verifiable

[CedricHermansBIT]

Problem

Atlas discovery cards had no structure-selection control. Researchers had to verify individual entries before hidden grouping controls appeared, making the Atlas look non-interactive.

Fix

• Add visible native selection checkboxes to each experimental PDB polymer-entity card, selected border, persistent count and a clear-selection action.
• Preserve selections across paginated RCSB result pages; cap cohorts at 12 verified entities.
• Add Verify selected structures, processing the chosen mmCIF/SIFTS requests sequentially to respect the browser's request-ID matching and avoid overwriting concurrent verification responses.
• Show clustering only when at least two selected entities have verified, unambiguous C-alpha coordinates. Selecting a different cohort invalidates stale geometry and Compare Groups handoff.
• Maintain individual mapping verification and downstream manual/automatic clustering behavior. No unexpected archive downloads from checkbox interaction.
• Update the in-app Atlas tutorial and methods documentation.
• Render selection toolbar separately so clicking a checkbox doesn't rebuild the scrollable result list and lose its position.

Regression testing

Update the existing real Chromium Atlas browser workflow to select two results across pagination, deselect/reselect, batch-verify them and finish the usual automatic/manual Atlas → Compare Groups flow. All eight standard CI workflows must be green before merging.

The deployed Shinylive site is not updated by merging this PR.

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