`benchmark-diff.sh` truncates fractional part of benchmark deltas

[gemshrine]

Problem

.github/benchmark-diff.sh formats numeric deltas with %d in the AWK report. Benchmark timings can contain fractional values, so this truncates the measured change before it is shown in the comparison table.

Steps to reproduce

Compare a baseline value of 10.4 with a current value of 11.1.

Actual result

The delta is printed as an integer (0), hiding the 0.7 ms regression.

Expected result

Keep the fractional precision of benchmark measurements when printing the delta and when deciding which changes to report.

[0crat]

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