refactor: revert canola namespace to oil and remove vim.g config (#120)

* refactor: revert module namespace from canola back to oil

Problem: the canola rename creates unnecessary friction for users
migrating from stevearc/oil.nvim — every `require('oil')` call and
config reference must change.

Solution: revert all module paths, URL schemes, autocmd groups,
highlight groups, and filetype names back to `oil`. The repo stays
`canola.nvim` for identity; the code is a drop-in replacement.

* refactor: remove `vim.g.oil` declarative config

Problem: the `vim.g.oil` configuration path was added prematurely.
It adds a second config entrypoint before the plugin has stabilized
enough to justify it.

Solution: remove `vim.g.oil` support from `plugin/oil.lua`,
`config.setup()`, docs, and tests. Users configure via
`require("oil").setup({})`.

lua/oil/loading.lua

@@ -0,0 +1,89 @@
+local util = require('oil.util')
+local M = {}
+
+local timers = {}
+
+local FPS = 20
+
+---@param bufnr integer
+---@return boolean
+M.is_loading = function(bufnr)
+  return timers[bufnr] ~= nil
+end
+
+local spinners = {
+  dots = { '⠋', '⠙', '⠹', '⠸', '⠼', '⠴', '⠦', '⠧', '⠇', '⠏' },
+}
+
+---@param name_or_frames string|string[]
+---@return fun(): string
+M.get_iter = function(name_or_frames)
+  local frames
+  if type(name_or_frames) == 'string' then
+    frames = spinners[name_or_frames]
+    if not frames then
+      error(string.format("Unrecognized spinner: '%s'", name_or_frames))
+    end
+  else
+    frames = name_or_frames
+  end
+  local i = 0
+  return function()
+    i = (i % #frames) + 1
+    return frames[i]
+  end
+end
+
+M.get_bar_iter = function(opts)
+  opts = vim.tbl_deep_extend('keep', opts or {}, {
+    bar_size = 3,
+    width = 20,
+  })
+  local i = 0
+  return function()
+    local chars = { '[' }
+    for _ = 1, opts.width - 2 do
+      table.insert(chars, ' ')
+    end
+    table.insert(chars, ']')
+
+    for j = i - opts.bar_size, i do
+      if j > 1 and j < opts.width then
+        chars[j] = '='
+      end
+    end
+
+    i = (i + 1) % (opts.width + opts.bar_size)
+    return table.concat(chars, '')
+  end
+end
+
+---@param bufnr integer
+---@param is_loading boolean
+M.set_loading = function(bufnr, is_loading)
+  if is_loading then
+    if timers[bufnr] == nil then
+      local width = 20
+      timers[bufnr] = vim.loop.new_timer()
+      local bar_iter = M.get_bar_iter({ width = width })
+      timers[bufnr]:start(
+        200, -- Delay the loading screen just a bit to avoid flicker
+        math.floor(1000 / FPS),
+        vim.schedule_wrap(function()
+          if not vim.api.nvim_buf_is_valid(bufnr) or not timers[bufnr] then
+            M.set_loading(bufnr, false)
+            return
+          end
+          local lines =
+            { util.pad_align('Loading', math.floor(width / 2) - 3, 'right'), bar_iter() }
+          util.render_text(bufnr, lines)
+        end)
+      )
+    end
+  elseif timers[bufnr] then
+    timers[bufnr]:close()
+    timers[bufnr] = nil
+  end
+end
+
+return M