refactor(spell): migrate to Lua, drop netrw dependency

Problem:
Spell file downloads relied on Vimscript and netrw (:Nread). If netrw is
disabled, downloads fail.

Solution:
Port the logic to Lua as `nvim.spellfile` and wire it via a Lua plugin that
handles `SpellFileMissing`. Use `vim.net.request()` with a timeout for HTTP,
prompt via `vim.fn.input` and report via `vim.notify`.

Closes #7189

runtime/plugin/nvim/spellfile.lua

@@ -0,0 +1,15 @@
+vim.g.loaded_spellfile_plugin = true
+
+--- Callback for SpellFileMissing: download missing .spl
+--- @param args { bufnr: integer, match: string }
+local function on_spellfile_missing(args)
+  local spellfile = require('nvim.spellfile')
+  spellfile.load_file(args.match)
+end
+
+vim.api.nvim_create_autocmd('SpellFileMissing', {
+  group = vim.api.nvim_create_augroup('nvim_spellfile', { clear = true }),
+  pattern = '*',
+  desc = 'Download missing spell files when setting spelllang',
+  callback = on_spellfile_missing,
+})