Problem:
The "tohtml" plugin is loaded by default.

Solution:
- Move it to `pack/dist/opt/nvim.tohtml/`, it is an "opt-in" plugin now.
- Document guidelines.
- Also revert the `plugin/` locations of `spellfile.lua` and `net.lua`.
  That idea was not worth the trouble, it will be too much re-education
  for too little gain.

Problem:
In autocmd examples, using "args" as the event-object name is vague and
may be confused with a user-command.

Solution:
Use "ev" as the conventional event-object name.

Problem:
When running ":edit <url>", filetype detection is not triggered.

Solution:
Run the autocmds in the filetypedetect group after loading the content.

Problem:
After fetching remote content from a URL and adding it to the buffer,
the buffer is marked as modified. This is inconsistent with the original
netrw behavior, and it causes problems with `:e` to refresh or `:q` as
it prompts for saving the file even if the user hasn't touched the
content at all.

Solution:
Mark the buffer as unmodified right after adding the remote content to
the buffer.

Problem:
- Exposing the raw config as table is a pattern not seen anywhere else
  in the Nvim codebase.
- Old spellfile.vim docs still available, no new documentation

Solution:
- Exposing a `config()` function that both acts as "getter" and "setter"
  is a much more common idiom (e.g. vim.lsp, vim.diagnostic).
- Add new documentation and link old docs to |spellfile.lua| instead of
  |spellfile.vim|.

- prefer `stdpath(data)/site/spell` instead of looking for random dirs in 'runtimepath'.
- drop unused functions `choose_directory`, `setup`, etc.

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

Problem:
Nvim depends on netrw to download/request URL contents.

Solution:
- Add `vim.net.request()` as a thin curl wrapper:
  - Basic GET with --silent, --show-error, --fail, --location, --retry
  - Optional `opts.outpath` to save to a file
  - Operates asynchronously. Pass an `on_response` handler to get the result.
- Add integ tests (requires NVIM_TEST_INTEG to be set) to test success
  and 404 failure.
- Health check for missing `curl`.
- Handle `:edit https://…` using `vim.net.request()`.

API Usage:
1. Asynchronous request:

    vim.net.request('https://httpbingo.org/get', { retry = 2 }, function(err, response)
      if err then
        print('Fetch failed:', err)
      else
        print('Got body of length:', #response.body)
      end
    end)

2. Download to file:

    vim.net.request('https://httpbingo.org/get', { outpath = 'out_async.txt' }, function(err)
      if err then print('Error:', err) end
    end)

3. Remote :edit integration (in runtime/plugin/net.lua) fetches into buffer:

    :edit https://httpbingo.org/get