fix(uv): prefer vim.uv with fallback

[khaneliman]

Summary

Prefer vim.uv throughout the active runtime and test code while preserving
Neovim 0.9 support.

Why

Neovim deprecates direct vim.loop use, while git-worktree.nvim supports
Neovim 0.9 and newer.

What changed

• Added file-local vim.uv or vim.loop bindings in the affected modules.
• Migrated 20 active current-directory and system-information calls.
• Updated the corresponding test assertions.

Testing

• StyLua and Luacheck passed.
• All 23 maintained tests passed on stable Neovim.
• Modern and fallback module-load probes passed.
• All changed files passed parse and diff checks.

Related Issue(s)

• None.