Allow trace goto at position zero

[liurenfeng94-ops]

Fixes a small trace navigation edge case.

TreeNode showed the goto button whenever a trace line had a position, including pos: 0, but the click handler used a truthy check and returned early for zero. This changes the guard to check only for undefined, so the start of a file remains navigable.

Validation:

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

[github-actions]

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[liurenfeng94-ops]

Thanks — I reviewed the text again. The actual fix is just the zero-position guard: pos: 0 should still navigate because the button is shown for defined positions.