Validate archive entries and stage inside the pack root

- Reject archive members that are absolute, escape via .., are special files, or link outside the destination, before anything is written

- Stage under the pack root so the swap is a same-filesystem rename, with an EXDEV copy fallback; propagate registry auth errors and validate dir_name

tests/manifest_spec.lua

@@ -140,4 +140,23 @@ describe('config', function()
     assert_nil(cfg)
     assert_match('no registry', err)
   end)
+
+  it('keeps a well-formed dir_name override', function()
+    local cfg = assert(config.normalize({
+      registry = reg,
+      plugins = { { name = 'oil.nvim', version = '2.11.0', dir_name = 'oil' } },
+    }))
+    assert_eq('oil', cfg.plugins[1].dir_name)
+  end)
+
+  it('rejects a dir_name that is not a single path component', function()
+    for _, bad in ipairs({ '../evil', '/abs', 'a/b', '..', '.', '' }) do
+      local cfg, err = config.normalize({
+        registry = reg,
+        plugins = { { name = 'a', version = '1', dir_name = bad } },
+      })
+      assert_nil(cfg, 'dir_name ' .. vim.inspect(bad) .. ' should be rejected')
+      assert_match('dir_name', err)
+    end
+  end)
 end)