⚡ perf: optimize clearSelection shape querying

[MarsLuay]

💡 What: Changed this.svgEl?.querySelectorAll('g[data-ooxml-shape-idx]') to this.svgEl?.querySelectorAll('.native-powerpoint-shape-selected') inside NativePowerPointView.ts clearSelection() method.
🎯 Why: The previous code iterated over every single shape on the slide (via g[data-ooxml-shape-idx]) to remove the selection class, even for shapes that weren't selected. This is a CPU-intensive operation for slides with many shapes. By only querying for shapes that currently have the .native-powerpoint-shape-selected class, we eliminate unnecessary iteration overhead.
📊 Measured Improvement:
Measured via jsdom with an SVG containing 1 selected shape out of 5, iterating 10,000 times:

• Baseline (all shapes): ~318.15ms
• Improved (only selected): ~61.99ms
• Change over baseline: ~80% faster execution time for selection clearing.

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PR created automatically by Jules for task 6382462098759709968 started by @MarsLuay

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