app/vlselect/logsql: fix silent log loss in live tail when entries share the same _time

[cuongleqq]

Fixes #1459

The problem:
The live tail dedup in getTailRows compared only timestamps, so any new row arriving at the same _time as the previous poll's last emitted row was dropped, even when content differed.

Solution:
Track xxhash fingerprints of rows at the boundary timestamp, so new rows are distinguished by content rather than timestamp alone.

[cubic-dev-ai]

No issues found across 3 files

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[cubic-dev-ai]

No issues found across 3 files

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