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

test/functional/lua/spellfile_spec.lua

@@ -0,0 +1,160 @@
+local n = require('test.functional.testnvim')()
+local t = require('test.testutil')
+
+local exec = n.exec
+local exec_lua = n.exec_lua
+local mkdir_p = n.mkdir_p
+local write_file = t.write_file
+local eq = t.eq
+
+describe('nvim.spellfile', function()
+  before_each(function()
+    n.clear()
+  end)
+
+  it('no-op when .spl and .sug already exist on rtp', function()
+    mkdir_p('Xplug/spell')
+    write_file('Xplug/spell/en_gb.utf-8.spl', 'dummy')
+    write_file('Xplug/spell/en_gb.utf-8.sug', 'dummy')
+    exec('set rtp+=' .. 'Xplug')
+
+    local out = exec_lua([[
+    local s = require('nvim.spellfile')
+
+    local my_spell = vim.fs.joinpath(vim.fn.fnamemodify('Xplug', ':p'), 'spell')
+    local old_access = vim.uv.fs_access
+    vim.uv.fs_access = function(p, mode)
+      return p == my_spell
+    end
+
+    local prompted = false
+    vim.fn.input = function() prompted = true; return 'n' end
+
+    local requests = 0
+    local orig_req = vim.net.request
+    vim.net.request = function(...) requests = requests + 1 end
+
+    s.load_file('en_gb')
+
+    vim.uv.fs_access = old_access
+    vim.net.request = orig_req
+
+    return { prompted = prompted, requests = requests }
+    ]])
+
+    eq(false, out.prompted)
+    eq(0, out.requests)
+  end)
+
+  it(
+    'downloads UTF-8 .spl to stdpath(data)/site/spell when no rtp spelldir; .sug 404 is non-fatal; reloads',
+    function()
+      mkdir_p('Xempty')
+      exec('set rtp+=' .. 'Xempty')
+
+      local out = exec_lua([[
+      local s = require('nvim.spellfile')
+
+      local data_root = 'Xdata'
+      vim.fn.stdpath = function(k)
+        assert(k == 'data')
+        return data_root
+      end
+
+      local old_access = vim.uv.fs_access
+      vim.uv.fs_access = function(_, _) return false end
+
+      vim.fn.input = function() return 'y' end
+
+      local reloaded = false
+      local orig_cmd = vim.cmd
+      vim.cmd = function(c)
+        if c:match('setlocal%s+spell!') then reloaded = true end
+        return orig_cmd(c)
+      end
+
+      local orig_req = vim.net.request
+      vim.net.request = function(url, opts, cb)
+        local name = url:match('/([^/]+)$')
+        if name and name:find('%.spl$') then
+          vim.fn.mkdir(vim.fs.dirname(opts.outpath), 'p')
+          vim.fn.writefile({'ok'}, opts.outpath)
+          cb(nil, { status = 200 })
+        else
+          cb(nil, { status = 404 })
+        end
+      end
+
+      s.load_file('en_gb')
+
+      local spl = vim.fs.joinpath(data_root, 'site', 'spell', 'en_gb.utf-8.spl')
+      local sug = vim.fs.joinpath(data_root, 'site', 'spell', 'en_gb.utf-8.sug')
+      local has_spl = vim.uv.fs_stat(spl) ~= nil
+      local has_sug = vim.uv.fs_stat(sug) ~= nil
+
+      vim.net.request = orig_req
+      vim.cmd = orig_cmd
+      vim.uv.fs_access = old_access
+
+      return { spl = has_spl, sug = has_sug, reloaded = reloaded }
+      ]])
+
+      eq(true, out.spl)
+      eq(false, out.sug)
+      eq(true, out.reloaded)
+    end
+  )
+
+  it('dual-fail: UTF-8 and ASCII 404 -> warn once, mark done, no reload', function()
+    mkdir_p('Xempty2')
+    exec('set rtp+=' .. 'Xempty2')
+
+    local out = exec_lua([[
+      local s = require('nvim.spellfile')
+
+      local data_root = 'Xdata2'
+      vim.fn.stdpath = function(k)
+        assert(k == 'data')
+        return data_root
+      end
+
+      local old_access = vim.uv.fs_access
+      vim.uv.fs_access = function(_, _) return false end
+      local old_stat = vim.uv.fs_stat
+      vim.uv.fs_stat = function(p) return old_stat and old_stat(p) or nil end
+
+      vim.fn.input = function() return 'y' end
+
+      local warns = 0
+      local orig_notify = vim.notify
+      vim.notify = function(_, lvl)
+        if lvl and lvl >= vim.log.levels.WARN then warns = warns + 1 end
+      end
+
+      local reloaded = false
+      local orig_cmd = vim.cmd
+      vim.cmd = function(c)
+        if c:match('setlocal%s+spell!') then reloaded = true end
+        return orig_cmd(c)
+      end
+
+      local orig_req = vim.net.request
+      vim.net.request = function(_, _, cb) cb(nil, { status = 404 }) end
+
+      local key = s.parse('zz').key
+      s.load_file('zz')
+      local done = (s.isDone(key)) == true
+
+      vim.net.request = orig_req
+      vim.notify = orig_notify
+      vim.cmd = orig_cmd
+      vim.uv.fs_access = old_access
+
+      return { warns = warns, done = done, reloaded = reloaded }
+    ]])
+
+    eq(1, out.warns)
+    eq(true, out.done)
+    eq(false, out.reloaded)
+  end)
+end)