fix: resolve relative PATH entries against the provided cwd - #128

Summary

• resolve ordinary relative PATH entries against the cwd supplied to which_in
• preserve tilde expansion and Windows drive-relative, root-relative, absolute, and UNC path semantics
• preserve platform-specific empty PATH behavior while using Sys::is_windows for custom system implementations
• add regression tests for tools, ./tools, ../tools, Unix cwd values containing :, empty PATH entries, and Windows-special path classification

Tests

• cargo test --workspace --all-features -- --include-ignored
• cargo test --workspace --no-default-features
• cargo clippy --workspace --all-targets --all-features -- -Dwarnings
• cargo fmt --all -- --check

Fixes #127