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.

lua/oil/adapters/test.lua

@@ -0,0 +1,96 @@
+local cache = require('oil.cache')
+local util = require('oil.util')
+local M = {}
+
+---@param url string
+---@param callback fun(url: string)
+M.normalize_url = function(url, callback)
+  callback(url)
+end
+
+local dir_listing = {}
+
+---@param url string
+---@param column_defs string[]
+---@param cb fun(err?: string, entries?: oil.InternalEntry[], fetch_more?: fun())
+M.list = function(url, column_defs, cb)
+  local _, path = util.parse_url(url)
+  local entries = dir_listing[path] or {}
+  local cache_entries = {}
+  for _, entry in ipairs(entries) do
+    local cache_entry = cache.create_entry(url, entry.name, entry.entry_type)
+    table.insert(cache_entries, cache_entry)
+  end
+  cb(nil, cache_entries)
+end
+
+M.test_clear = function()
+  dir_listing = {}
+end
+
+---@param path string
+---@param entry_type oil.EntryType
+---@return oil.InternalEntry
+M.test_set = function(path, entry_type)
+  if path == '/' then
+    return {}
+  end
+  local parent = vim.fn.fnamemodify(path, ':h')
+  if parent ~= path then
+    M.test_set(parent, 'directory')
+  end
+  parent = util.addslash(parent)
+  if not dir_listing[parent] then
+    dir_listing[parent] = {}
+  end
+  local name = vim.fn.fnamemodify(path, ':t')
+  local entry = {
+    name = name,
+    entry_type = entry_type,
+  }
+  table.insert(dir_listing[parent], entry)
+  local parent_url = 'oil-test://' .. parent
+  return cache.create_and_store_entry(parent_url, entry.name, entry.entry_type)
+end
+
+---@param name string
+---@return nil|oil.ColumnDefinition
+M.get_column = function(name)
+  return nil
+end
+
+---@param bufnr integer
+---@return boolean
+M.is_modifiable = function(bufnr)
+  return true
+end
+
+---@param action oil.Action
+---@return string
+M.render_action = function(action)
+  if action.type == 'create' or action.type == 'delete' then
+    return string.format('%s %s', action.type:upper(), action.url)
+  elseif action.type == 'move' or action.type == 'copy' then
+    return string.format('  %s %s -> %s', action.type:upper(), action.src_url, action.dest_url)
+  else
+    error('Bad action type')
+  end
+end
+
+---@param action oil.Action
+---@param cb fun(err: nil|string)
+M.perform_action = function(action, cb)
+  cb()
+end
+
+---@param bufnr integer
+M.read_file = function(bufnr)
+  -- pass
+end
+
+---@param bufnr integer
+M.write_file = function(bufnr)
+  -- pass
+end
+
+return M