⚡ Bolt: Replace Array.from in CSV indexing to eliminate closure overhead

[dieterolson]

• 💡 What: Replaced Array.from({ length: n }) with a pre-allocated new Array(n) and a standard for loop for default CSV index generation in DataExplorer.tsx.
• 🎯 Why: In V8, Array.from on an object with a length property creates an iterator and executes a callback for every element. For large datasets (like importing 100k+ row CSVs), this causes significant CPU and GC overhead compared to a basic loop.
• 📊 Impact: Substantially reduces main-thread blocking time and garbage collection pressure when loading large CSV files without explicit time columns.
• 🔬 Measurement: Profile the main thread in Chrome DevTools while importing a 100k+ row CSV with no time column; observe the reduced execution time and memory allocation during the buildDataset preparation phase.

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PR created automatically by Jules for task 8732981744710152070 started by @dieterolson

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[dieterolson]

Consolidated into #4408. Closing to reduce CI/CD runner load.