fix(cli): handle singular counts in scan summary

[vhmns14]

Description

Fixes #99.

Handles singular counts appropriately in diskern scan:

• Summary headline now outputs 1 finding (singular) and 1 duplicate set (singular) instead of 1 findings and 1 duplicate sets.
• Duplicate files section now outputs 1 set instead of 1 sets.
• Added a plural helper right below human_bytes to keep call sites clean and consistent.

Testing

• Added unit test for plural in crates/diskern-cli/src/main.rs.
• Added regression integration tests in crates/diskern-cli/tests/singular_summary.rs testing single duplicate set and single finding scans.
• Verified all tests pass with cargo test -p diskern-cli.
• Verified formatting with cargo fmt --check.

[Muawiya-contact]

@vhmns14 Welcome, and thanks for picking this up! 🙌

This is exactly the shape I hoped for. The helper next to human_bytes is the right home, you caught all three call sites, and folding the existing ternary in print_findings into it means there's now one place to get this wrong instead of three.

Nice touch testing plural(0) — that's the case people forget.

Ran it here and approved CI: fmt, clippy and all 69 tests pass on Linux, macOS and Windows. The integration tests depend on where tempdir() lands, which differs on all three, so I wanted to see that go green before approving. It did.

Approving. If you fancy another, the issues here are labelled by difficulty — level: beginner is the small self-contained ones. Coding Moves has a few other repos too if Rust isn't your usual thing. And if you spot something off anywhere, opening an issue is just as welcome as a PR.

If you're enjoying the project, a ⭐ helps people find it. Hope to see you around 🚀