Add unit tests for disk calculation logic (PR #364 test plan)

[Copilot]

PR #364 refactors disk space reporting (SI vs IEC divisors, physical disk deduplication). The disk calculation logic was untestable because it was inlined against live OS APIs. This PR extracts it into a pure, testable function and adds coverage.

Refactor: extract calc_hdd_stats

Pulls the disk aggregation logic out of sample() into a standalone function:

pub(crate) fn calc_hdd_stats(disks: &[DiskCalcInput], si: bool, is_windows: bool) -> (u64, u64)

• DiskCalcInput captures only name, fs_type, total, avail — no sysinfo dependency
• Divisor is now explicit: 1_000_000 (SI/macOS) vs 1_048_576 (IEC/Linux+Windows) based on stat.si
• Dedup HashSet allocated only on non-Windows paths
• Removes the now-unused unit: u64 variable from sample()

Tests added (sys_info::tests, 10 cases)

Category	Tests
Unit conversion	SI (÷1 000 000) and IEC (÷1 048 576) divisors produce correct MB/MiB values
FS filtering	Unknown types (tmpfs, devtmpfs) excluded; all-unknown → zero
Dedup (non-Windows)	Same disk name counted once; distinct names both counted
Windows	Dedup skipped — duplicate names counted independently
Edge cases	Empty list, fully-used disk, case-insensitive FS match (EXT4 → ext4)

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