pkg/query: Fix jitter handling

[brancz]

Whether a sample follows another can only be determined by the previous timestamp plus the time that sample represents, which is the cumulative, not the duration.

[alwaysmeticulous]

✅ Meticulous spotted 0 visual differences across 412 screens tested: view results.

Meticulous evaluated ~4 hours of user flows against your PR.

_{Expected differences? Click here. Last updated for commit 5bd0205. This comment will update as new commits are pushed.}