fix(man): use direct lookup without manpath #40421

Problem:
On NetBSD, `man -w open` can return the exact manpage path, but `:Man`
may still fail when man directories cannot be discovered from `manpath
-q`, bare `man -w`, or `$MANPATH`.

Solution:
Fall back to the direct manpage lookup when directory discovery fails.
Add a test for resolving `open(2)` through `goto_tag()` without manpath
data.

runtime/lua/man.lua

@@ -510,6 +510,11 @@ local function get_paths(name, sect)
     or vim.env.MANPATH
 
   if not mandirs_raw then
+    -- Fall back to direct lookup ("man -w [sect] name"). NetBSD man lacks "-w". #25919
+    local ok, path = pcall(M._find_path, name, sect)
+    if ok and path then
+      return { path }
+    end
     return {}, "Could not determine man directories from: 'man -w', 'manpath' or $MANPATH"
   end