chore: improve hyperfine maintenance path

[vip892766gma]

Summary:

• Add edge-case unit tests for the Second/Unit formatting and min/max helpers: cover zero, very small (sub-microsecond) and very large second values for unit selection/formatting, and empty-slice / single-element / NaN-handling behavior for min_max helpers, asserting current documented behavior to lock it in.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.