fix: prevent scientific notation for exact powers of 10

[underwoo]

Fixes #1

format(FSize(n * 1024), "K") now returns "10" instead of "1e+01" for exact powers of 10.

Root cause: math.ceil(math.log10(n)) produces precision equal to the exponent, causing Python's g format to use scientific notation.

Fix: Changed to math.floor(math.log10(n)) + 1 so precision always exceeds the exponent by at least 1.

Verification:

• pytest: 50/50 passed
• mypy: clean
• pylint: clean

Closes #1