fix: prevent crash with render-markdown.nvim on new file diff accept

[bluzername]

Problem

Neovim crashes with exit code 139 (segfault) when you accept a new file diff by pressing :w while render-markdown.nvim is installed. Only happens with new files, existing file diffs work fine.

I was losing my neovim session every time Claude suggested a new markdown file and I tried to accept it. Very annoying because you lose all your buffers.

Root Cause

After the BufWriteCmd callback finishes _resolve_diff_as_saved() and returns, Neovim does a post-write redraw. At this point the buffer is still in diff mode, and render-markdown.nvim tries to render it during the redraw cycle. For new files (where no old file existed on disk), this combination triggers a segfault.

The issue reporter (@tomerlevy1) did excellent debugging with breadcrumb logging and confirmed that all Lua code completes successfully - the crash happens after the callback returns control back to Neovim's C code.

Fix

Added pcall(vim.cmd, "diffoff") after _resolve_diff_as_saved() but before return true in the BufWriteCmd callback. This explicitly turns off diff mode before Neovim does the post-write redraw, so when render-markdown.nvim runs during redraw the buffer is no longer in diff state.

Using pcall because if diffoff fails for whatever reason we still want the callback to complete normally.

callback = function()
    M._resolve_diff_as_saved(tab_name, new_buffer)
    pcall(vim.cmd, "diffoff")  -- prevents segfault with render-markdown.nvim
    return true
end,

Testing

• Lua syntax verified
• This is the exact fix that the issue author tested and confirmed prevents the crash
• The change is minimal (one line) and uses pcall so it cannot introduce new errors
• Only affects the BufWriteCmd callback for new file diffs, existing behavior is unchanged

Closes #218