Add multi-select node comparison panel

[lowhung]

Summary

Enable multi-selection of nodes for side-by-side metric comparison.

Features

• Hold Shift+Click to multi-select nodes
• Comparison panel showing metrics side-by-side
• Relative metrics (faster/slower than average)
• Correlation detection (modules with correlated behavior)
• Difference highlighting (show which metrics differ most)

UI Panel Example

Selected: 3 modules [block_unpacker, accounts_state, validator]

Metric           | block_unpacker | accounts_state | validator    | Avg
Throughput       | 1.2k/s (↑110%) | 890/s (↓ 10%) | 1.1k/s (↑8%) | 1.0k/s
Health Score     | 92 (✓)         | 78 (!)        | 95 (✓)       | 88
Consumer Lag     | 150ms          | 2.3s (!)      | 180ms        | 0.9s

Benefits

• Compare similar modules easily
• Identify outliers
• Correlate behavior across modules