fix: close side buffers when splitting current buffer

[pradyuman]

Hi! I ran into an issue where running plain :vsplit wasn't hiding Zen's side buffers. This PR fixes that by handling WinNew through the same split logic as the existing BufWinEnter path (alongside a regression test).

zen.nvim has been great otherwise -- thanks for building it!

[Copilot]

🟢 Approval recommended

The change is small, consistent with existing autocmd logic, and includes a focused regression test for the reported scenario.

Pull request overview

This PR fixes a Zen side-buffer visibility bug when users split the current buffer via plain :vsplit, by ensuring the same “multiple vsplits” closure logic runs on WinNew (not just BufWinEnter). It also adds a regression test to cover the previously failing scenario.

Changes:

• Extend the existing autocmd logic to also handle WinNew, ensuring side buffers close when splitting the current buffer.
• Add a regression test for :vsplit with no target file (splitting the current buffer).

File summaries

File	Description
lua/zen/init.lua	Adds WinNew handling to trigger the existing split-detection/side-buffer close logic for current-buffer splits.
tests/test_splits.lua	Adds a regression test verifying :vsplit (without a file arg) closes Zen side buffers.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[sand4rt]

Thanks!