refactor: replace win_execute string injection in _adjust_scroll

[leolaurindo]

Summary

This is a behavior-preserving refactor of the internal _adjust_scroll implementation.

_adjust_scroll handles deleted virtual lines at the edges of a buffer:

• Deleted lines before the first buffer line use virt_lines_above and require topfill.
• Deleted lines after the final buffer line require scrolling past EOF so they become visible.

The current implementation performs these operations through dynamically constructed Ex-command strings passed to win_execute.

What changed

Before, the top-of-file case built and evaluated Lua source code:

vim.fn.win_execute(
  winid,
  "lua vim.fn.winrestview({topfill=" .. count .. "})"
)

It is now a direct window-scoped function call:

vim.api.nvim_win_call(winid, function()
  vim.fn.winrestview({ topfill = count })
end)
The bottom-of-file case previously built a normal-mode command string:
vim.fn.win_execute(winid, "normal! " .. needed .. "\5")

It now uses the documented window-call API:

vim.api.nvim_win_call(winid, function()
  vim.cmd.normal(needed .. "\5")
end)

Why

The previous code constructed executable Lua/Vimscript as strings. That makes the operation:

• Harder for static analysis and linters to inspect
• More fragile during future refactors
• Dependent on manual string construction and escaping
• Less explicit about which window receives the operation

nvim_win_call() directly expresses the intended operation: execute this function with a specific window as the current window.

The CTRL-E bottom scroll is intentionally retained. winrestview({ topline = ... }) clamps at the final buffer line and cannot reveal virtual lines positioned past EOF. The normal-mode scroll remains necessary for that case.

User-visible behavior

None intended. This is an internal refactor only.

The existing scroll tests pass unchanged:

Success: 23

Failed: 0

Errors: 0

The combined demo branch containing this and the other proposed changes is available at:
https://github.com/leolaurindo/inline-diff.nvim/tree/dev/all