fix: carry filesize suffix when mantissa rounds up to base

[patchwright]

Problem

All three public functions (traditional, binary, decimal) can produce impossible output like "1,024.0 KB" instead of "1.0 MB" for values just below a unit boundary.

Root cause: _to_str picks the suffix from the unrounded value, then formats the mantissa — which can round up to base, producing an impossible result.

filesize.decimal(999_999)        # '1,000.0 kB'  ← should be '1.0 MB'
filesize.traditional(1024**2-1)  # '1,024.0 KB'  ← should be '1.0 MB'
filesize.binary(1024**2-1)       # '1,024.0 KiB' ← should be '1.0 MiB'

Fix

Convert suffixes to a list before the loop. After computing the mantissa, round and compare against base. If the rounded value hits base, step up one suffix and recompute:

mantissa = base * size / unit
if round(mantissa, 1) >= base and i - 1 < len(suffixes):
    suffix = suffixes[i - 1]
    mantissa = size / unit

Math: when the current unit is base**i, dividing size by that same unit gives the value in the next suffix (whose denominator is base**(i+1), which divides into base * size / base**(i+1) = size / base**i = size / unit).

Also removes the existing TODO comment that flagged this exact issue.

Tests

Added test_rollover_decimal, test_rollover_traditional, and test_rollover_binary covering the unit boundaries for all three functions.