Round the arrival timeline axis labels to whole milliseconds

[samcm]

The timeline axis bounds are computed as fractions of the observation spread, so the left label could render float noise like 220.64999999999998ms. fmtAxis now rounds to whole milliseconds before formatting.

https://claude.ai/code/session_01QTfZjLgwhNK5i4At7BfrBR

[redpandabot]

The PR rounds axis label values to whole milliseconds in fmtAxis before formatting, fixing float-noise labels like 220.64999999999998ms on the timeline axis bounds. The change is display-only: all four fmtAxis call sites feed axis geometry or whole-number steps, pos()/span are untouched, inputs are always non-negative (no -0 edge), and rounding can only shorten labels, which strictly helps the axisHeader label-overlap heuristics.

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_{Reviewed 1 changed file(s) @ f7472e61 — no blocking issues found.
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