Validate file-perf I/O depth and block size

[fallintoplace]

Summary

• Reject --iodepth 0 before starting the benchmark.
• Reject block sizes outside the uint32_t range before narrowing.
• Add regression coverage for both invalid CLI values.

--iodepth 0 leads to invalid slot access and a modulo-by-zero path. A size such as 4g was narrowed to zero before the existing block-size checks.

Tests

• git diff --check
• ./bb fmt --check
• Targeted x86-64 Linux build of file-perf
• Targeted CTest cases for zero I/O depth and oversized block size

[praktika-gh]

Workflow [PR], commit [c474a8e]

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Code Review

Result: ✅ No issues found

What changed: Validates that file-perf I/O depth is nonzero and block size fits in a nonzero uint32_t before benchmark setup. It also detects overflow in suffixed size parsing and adds CTest regression coverage for invalid values.

No issues found. The previously reported suffix-multiplication overflow is now checked before multiplication and covered by a regression test.

Investigation: 3/13 rounds, 16 tool calls.