fix(man.lua): cache pages until width changes #40607

Problem: :Man unloads hidden buffers, so reopening large pages reruns rendering
even when the width is unchanged.

Solution: keep regular man buffers loaded and refresh them only when the
effective width changes.

(cherry picked from commit a6827be98f)

runtime/plugin/man.lua

@@ -37,3 +37,14 @@ vim.api.nvim_create_autocmd('BufReadCmd', {
     end
   end,
 })
+
+vim.api.nvim_create_autocmd('BufWinEnter', {
+  group = augroup,
+  pattern = 'man://*',
+  callback = function()
+    local err = require('man').refresh_page()
+    if err then
+      vim.notify('man.lua: ' .. err, vim.log.levels.ERROR)
+    end
+  end,
+})