Hide shell functions from _G using upvalues on the main chunk

Store __command, __interactive, __getenv, __setenv as upvalues
populated by lua_load() from the registry, keeping them invisible
to user code while accessible to the parser's codegen.

testes/lush/builtins.lua

@@ -3,13 +3,10 @@
 
 print "testing shell builtins"
 
--- === __builtins table ===
+-- === __builtins is NOT in _G (internal, stored in registry) ===
 
 do
-  assert(type(__builtins) == "table")
-  assert(type(__builtins.cd) == "function")
-  assert(type(__builtins.exec) == "function")
-  assert(type(__builtins.umask) == "function")
+  assert(__builtins == nil, "__builtins should not be in _G")
 end
 
 
@@ -117,9 +114,9 @@ end
 
 -- === exec ===
 
--- exec with no command returns error
+-- exec with no command returns error (tested via backtick)
 do
-  local r = __builtins.exec("exec")
+  local r = `exec`
   assert(r.code == 1)
   assert(r.stderr:find("command required"),
          "expected 'command required', got: " .. r.stderr)
@@ -127,7 +124,7 @@ end
 
 -- exec nonexistent command returns error
 do
-  local r = __builtins.exec("exec", "nonexistent_cmd_xyz_999")
+  local r = `exec nonexistent_cmd_xyz_999`
   assert(r.code == 1)
   assert(r.stderr:find("exec:"),
          "expected exec error, got: " .. r.stderr)
@@ -140,21 +137,4 @@ do
 end
 
 
--- === builtins are overridable ===
-
-do
-  local old_cd = __builtins.cd
-  local called = false
-  __builtins.cd = function(...)
-    called = true
-    return old_cd(...)
-  end
-  local before = `pwd`.stdout:gsub("\n$", "")
-  `cd /tmp`
-  assert(called, "custom cd was not called")
-  __builtins.cd = old_cd
-  `cd ${before}`
-end
-
-
 print "OK"