feat(ui): use vim.ui.select for :oldfiles, :recover

Problem:
followup to 55ceb314ca #39478
`:oldfiles` and swapfile `:recover` do not delegate to `vim.ui.select`.

Solution:
- Delegate to `vim.ui.select`.
- Fix a long-standing `recover_names` bug where `concat_fnames(dir_name,
  files[i], true)` produced malformed `<dir>//<dir>/<file>` paths (also
  fixes `swapfilelist()`).

test/functional/lua/ui_select_spec.lua

@@ -6,10 +6,48 @@ local clear = n.clear
 local exec_lua = n.exec_lua
 local api = n.api
 local eq = t.eq
+local neq = t.neq
 local write_file = t.write_file
 
 before_each(clear)
 
+--- Mock async vim.ui.select impl. Imitates fzf-lua/telescope/snacks: opens
+--- a transient floating window, then schedules on_choice to fire on the next
+--- event-loop tick (rather than synchronously).
+---
+--- Sets `_G._captured` so tests can inspect what was passed to vim.ui.select.
+--- @param pick integer|nil 1-based index to "pick" (nil cancels).
+local function setup_async_picker(pick)
+  exec_lua(function()
+    _G._captured = nil
+    --- @diagnostic disable-next-line: duplicate-set-field
+    vim.ui.select = function(items, opts, on_choice)
+      _G._captured = { items = items, opts = opts }
+      -- Open a floating window like a real picker would.
+      local buf = vim.api.nvim_create_buf(false, true)
+      local win = vim.api.nvim_open_win(buf, false, {
+        relative = 'editor',
+        row = 1,
+        col = 1,
+        width = 30,
+        height = math.min(#items, 5),
+      })
+      _G._captured.win = win
+      -- Defer the choice so the wait actually has to pump events.
+      vim.defer_fn(function()
+        if vim.api.nvim_win_is_valid(win) then
+          vim.api.nvim_win_close(win, true)
+        end
+        if pick then
+          on_choice(items[pick], pick)
+        else
+          on_choice(nil, nil)
+        end
+      end, 30)
+    end
+  end, pick)
+end
+
 describe('vim.ui.select()', function()
   it('can select an item', function()
     local result = exec_lua [[
@@ -165,4 +203,151 @@ describe('vim.ui.select()', function()
       eq('helo', api.nvim_buf_get_lines(0, 0, -1, false)[1])
     end)
   end)
+
+  -- The selection step blocks the C caller via vim.wait(). Async pickers
+  -- (fzf-lua, telescope, snacks, …) open a transient window and call on_choice
+  -- on a later event-loop tick. These tests exercise the wait+resume path for
+  -- each integration. If the C caller doesn't allow the picker to repaint or
+  -- pump events, these will hang or fail.
+  describe('async picker', function()
+    it('z= dispatches selection from a deferred callback', function()
+      api.nvim_set_option_value('spell', true, {})
+      api.nvim_set_option_value('spelllang', 'en_us', {})
+      api.nvim_buf_set_lines(0, 0, -1, false, { 'helo' })
+
+      setup_async_picker(1)
+      exec_lua(function()
+        vim.cmd('normal! gg0z=')
+      end)
+
+      neq('helo', api.nvim_buf_get_lines(0, 0, -1, false)[1])
+      local kind = exec_lua([[return _G._captured and _G._captured.opts.kind]])
+      eq('spell', kind)
+    end)
+
+    it(':tselect dispatches selection from a deferred callback', function()
+      write_file('XselTagA.c', 'int foo;\n')
+      write_file('XselTagB.c', 'int foo = 1;\n')
+      finally(function()
+        os.remove('XselTagA.c')
+        os.remove('XselTagB.c')
+        os.remove('XselTags')
+      end)
+      write_file(
+        'XselTags',
+        '!_TAG_FILE_FORMAT\t2\t/extended format/\n'
+          .. 'foo\tXselTagA.c\t/^int foo;$/;"\tv\n'
+          .. 'foo\tXselTagB.c\t/^int foo = 1;$/;"\tv\n'
+      )
+      api.nvim_set_option_value('tags', 'XselTags', {})
+
+      setup_async_picker(2)
+      exec_lua(function()
+        vim.cmd('tselect foo')
+      end)
+
+      eq('XselTagB.c', api.nvim_eval('expand("%:t")'))
+      local kind = exec_lua([[return _G._captured and _G._captured.opts.kind]])
+      eq('tag', kind)
+    end)
+
+    --- Mock fzf-lua-style picker: opens a floating window with a *terminal*
+    --- buffer running a small shell command. When the command exits we treat
+    --- the user as having "picked" `pick`. This more closely exercises the
+    --- code paths that block real terminal-based pickers in ex-command
+    --- contexts (RedrawingDisabled, mode dispatch, terminal_loop, …).
+    local function setup_term_picker(pick)
+      exec_lua(function()
+        _G._captured = nil
+        --- @diagnostic disable-next-line: duplicate-set-field
+        vim.ui.select = function(items, opts, on_choice)
+          _G._captured = { items = items, opts = opts }
+          local buf = vim.api.nvim_create_buf(false, true)
+          local win = vim.api.nvim_open_win(buf, true, {
+            relative = 'editor',
+            row = 1,
+            col = 1,
+            width = 30,
+            height = math.min(#items, 5),
+          })
+          -- Sleep briefly to mimic an interactive terminal session, then exit.
+          vim.fn.jobstart({ 'sh', '-c', 'sleep 0.05' }, {
+            term = true,
+            on_exit = function()
+              if vim.api.nvim_win_is_valid(win) then
+                vim.api.nvim_win_close(win, true)
+              end
+              if pick then
+                on_choice(items[pick], pick)
+              else
+                on_choice(nil, nil)
+              end
+            end,
+          })
+        end
+      end, pick)
+    end
+
+    it('z= dispatches selection from a terminal-based picker', function()
+      api.nvim_set_option_value('spell', true, {})
+      api.nvim_set_option_value('spelllang', 'en_us', {})
+      api.nvim_buf_set_lines(0, 0, -1, false, { 'helo' })
+
+      setup_term_picker(1)
+      exec_lua(function()
+        vim.cmd('normal! gg0z=')
+      end)
+
+      neq('helo', api.nvim_buf_get_lines(0, 0, -1, false)[1])
+    end)
+
+    it(':tselect dispatches selection from a terminal-based picker', function()
+      write_file('XselTagA.c', 'int foo;\n')
+      write_file('XselTagB.c', 'int foo = 1;\n')
+      finally(function()
+        os.remove('XselTagA.c')
+        os.remove('XselTagB.c')
+        os.remove('XselTags')
+      end)
+      write_file(
+        'XselTags',
+        '!_TAG_FILE_FORMAT\t2\t/extended format/\n'
+          .. 'foo\tXselTagA.c\t/^int foo;$/;"\tv\n'
+          .. 'foo\tXselTagB.c\t/^int foo = 1;$/;"\tv\n'
+      )
+      api.nvim_set_option_value('tags', 'XselTags', {})
+
+      setup_term_picker(2)
+      exec_lua(function()
+        vim.cmd('tselect foo')
+      end)
+
+      eq('XselTagB.c', api.nvim_eval('expand("%:t")'))
+    end)
+
+    it(':browse oldfiles dispatches selection from a deferred callback', function()
+      finally(function()
+        os.remove('XselOldA')
+        os.remove('XselOldB')
+      end)
+      write_file('XselOldA', 'a\n')
+      write_file('XselOldB', 'b\n')
+      local cwd = exec_lua([[return vim.uv.cwd()]])
+
+      setup_async_picker(2)
+      exec_lua(function(cwd_)
+        -- v:oldfiles is normally populated via shada; inject directly for the test.
+        vim.v.oldfiles = { cwd_ .. '/XselOldA', cwd_ .. '/XselOldB' }
+        vim.cmd('browse oldfiles')
+        -- :browse oldfiles is async — wait for on_choice to fire and edit the file.
+        vim.wait(1000, function()
+          return vim.fn.expand('%:t') == 'XselOldB'
+        end)
+      end, cwd)
+
+      eq('XselOldB', api.nvim_eval('expand("%:t")'))
+      local kind = exec_lua([[return _G._captured and _G._captured.opts.kind]])
+      eq('oldfiles', kind)
+    end)
+  end)
 end)