Improve bundle size chart to show differences between versions

[Copilot]

The bundle size chart now displays the differences between consecutive versions instead of absolute values, making it much easier to see actual changes between Rolldown versions.

Problem

The original bundle size chart showed absolute values (e.g., 571,470 bytes vs 571,458 bytes) which made it extremely difficult to see the actual changes between versions. Users had to manually calculate differences to understand performance improvements or regressions.

Solution

Transformed the bundle size data to show differences between consecutive versions with enhanced visual indicators:

• Gray bars for the baseline version (first version, 0 change)
• Green bars for bundle size decreases (negative values)
• Red bars for bundle size increases (positive values)
• Enhanced tooltips showing both the change amount and actual size transition (e.g., "-12 bytes (571,470 → 571,458)")

Example

With the test data, the chart now clearly shows:

• v7.1.0: Baseline (0 change)
• v7.1.1: -12 bytes decrease
• v7.1.2: 0 bytes change
• v7.1.3: -834 bytes decrease
• v7.1.4: 0 bytes change

This makes it immediately obvious that v7.1.3 had the most significant bundle size improvement, which was invisible in the original absolute value chart.

Fixes #36.

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