Fix trace tree root duration

[liurenfeng94-ops]

Fixes the root duration computed by toTree.

The code compared event durations while tracking the root duration, but assigned line.ts when it found a larger duration. That mixes timestamp and duration units. This now tracks the latest finite event end timestamp (ts + dur) so the synthetic root duration covers the loaded trace range.

Validation:

• pnpm vitest run test/trace-duration.test.ts
• pnpm vitest run
• pnpm typecheck
• pnpm exec eslint .
• pnpm build

[github-actions]

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