Add hot path trace visualization

[omercelik]

Summary

Adds a compact hot-path visualization for trace tree nodes so users can inspect the longest-running child chain without manually expanding deep trees.

• add a hotPathById webview message and host-side response
• compute the longest-duration child path with duration, type counts, child counts, and source position metadata
• render the path as an inline bar chart from each trace node
• add unit coverage for the hot-path selection logic

Closes #46.

Verification

• pnpm typecheck
• pnpm build
• CI=1 pnpm test