feat: Add CloudWatch multi-metric comparison overlays

[nathanhuh]

Summary

Add optional multi-metric comparison support after the single-series CloudWatch metrics flow is stable.

Details

• allow selecting and overlaying a small set of related metric series on one chart
• preserve readable terminal rendering with a legend or other low-noise series identification
• keep the comparison workflow scoped to practical side-by-side operational use cases
• explicitly treat this as a follow-up to the base viewer and controls work, not part of the initial metrics feature
• add focused unit tests

Checklist

• Add multi-series selection support
• Add readable in-terminal overlay rendering
• Add legend or equivalent series identification
• Add tests

Follows #125, depends on #126, and supersedes part of #30.

[nathanhuh]

@Unic-bot: assign me

[github-actions]

@nathanhuh has been assigned to this issue.