Add highlighted cell syncing for Landscape

[cornhundred]

Summary

• add a selected_cells trait and helper method so notebooks can send highlighted cell ids to the Landscape widget
• propagate selected cell selections to the frontend observable store and refresh deck.gl layers with highlight-aware colors
• embed cell ids in scatter data so highlighted cells render brightly while other points are dimmed

Testing

• Not run (not requested)

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Codex Task

[Copilot]

Pull request overview

This PR adds cell highlighting functionality to the Landscape widget, enabling notebooks to visually emphasize specific cells while dimming others. The implementation synchronizes selected cell IDs between Python and JavaScript, updates the rendering pipeline to apply highlight-aware colors, and embeds cell identifiers in scatter plot data.

Key Changes:

• Added selected_cells trait and highlight_cells() method to the Python widget for cell selection management
• Implemented frontend state management for highlighted cells with observable store integration and layer refresh logic
• Modified cell rendering to display highlighted cells in bright blue while dimming non-highlighted cells to low opacity

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/celldega/viz/widget.py	Adds selected_cells trait and highlight_cells() helper method for syncing cell IDs from notebook to widget
js/viz/landscape_ist.js	Implements highlighted cell state management, model synchronization listener, and layer refresh mechanism
js/obs_store/obs_store.js	Adds selected_cells observable to the store for tracking highlighted cells
js/deck-gl/layers/cell_layer.js	Updates color function to handle highlighting logic, embeds cell names in scatter data, and adds selection trigger

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