Fix format() printing 1024 in the auto-selected unit

[afonsojanu]

format() auto-picks a unit for a byte count by comparing the raw value against each threshold (>= 1024 for KB, >= 1024**2 for MB, and so on), then divides and rounds to decimalPlaces. That rounding step can push a value that was just under a threshold back up to (or past) 1024 in the unit that already got picked:

require('bytes').format(1048575.995, { decimalPlaces: 2 })
// => '1024.00KB'   (should be '1MB')

require('bytes').format(1073741823.9999, { decimalPlaces: 2 })
// => '1024.00MB'   (should be '1GB')

1024.00KB and 1MB are the same amount, but printing it in the unit that was supposed to be "too small for this" defeats the whole point of the auto-selection, and reads as if it just failed to notice it crossed the boundary.

The fix re-checks after rounding: if the formatted magnitude still reads >= 1024 and there's a bigger unit available, it moves up and reformats against that unit instead (looping in case one bump isn't enough, though in practice one is). This only applies when the unit was auto-selected — passing an explicit unit option is a deliberate choice and is left alone, so bytes.format(1048575.995, { decimalPlaces: 2, unit: 'KB' }) still prints 1024KB.

Added a test ("Should never round the displayed value up to the next unit's threshold") covering the KB/MB/GB/TB/PB boundaries, negative values, and the explicit-unit case. Confirmed it fails on the unpatched index.js and passes with the fix; npm test (mocha) and npm run lint (eslint) both still pass with no new warnings.

[afonsojanu]

Closing this one too, same reason: starting fresh and clearing out everything I had open.