feat: add mount point exclusion

[wakamex]

Why

I use dust to list immediate project sizes before moving directories between disks, and I want mounted directories omitted explicitly.

For my current tree, -x produces the same result because the mounted child is on another filesystem. But -x checks filesystem device metadata throughout the walk, while an explicit mount-point filter can skip known mount targets directly. It also covers same-filesystem bind mounts, which device-ID filtering cannot distinguish from ordinary directories.

Paired local timings with byte-identical JSON output:

• dust -d 1 -D -x: 28.7 seconds
• dust -d 1 -D --exclude-mounts: 20.9 seconds

What changed

--exclude-mounts skips mount points below each supplied path while still allowing a supplied path that is itself mounted.

On Linux it reads /proc/self/mountinfo, which covers same-device bind mounts. Other platforms use the existing sysinfo disk mount-point inventory. The option is also available as exclude-mounts=true in the config file.

The README, sample config, man page, and generated shell completions are updated.

Validation

• cargo fmt --all -- --check
• cargo test --all-targets - 80 tests passed
• cargo clippy -- -D warnings
• cargo check --target x86_64-pc-windows-gnu
• Isolated Linux test with source and bind target on the same device

OpenAI Codex assisted with code generation, testing, and review. I reviewed and validated the final patch, and the commit includes an Assisted-by trailer.

[wakamex]

After checking my actual mount layout more carefully, -x already has the semantics I need because the only descendant mount is on a different filesystem. The performance gap that led me here comes from redundant metadata work in the current -x path, and #575 already addresses that. Closing this in favor of #575. Thanks.