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/registry_spec.lua

@@ -74,6 +74,28 @@ describe('url escape hatch', function()
     assert_eq('https://h/x.zip', resolved.url)
     assert_eq('x.zip', resolved.filename)
   end)
+
+  it('still applies the registry auth headers', function()
+    vim.env.VIMPACK_TEST_TOKEN = 'sekret'
+    local resolved = assert(
+      registry.resolve(
+        { name = 'x', version = '1', url = 'https://h/x.zip' },
+        { auth = { env = 'VIMPACK_TEST_TOKEN' } }
+      )
+    )
+    assert_eq('Bearer sekret', resolved.headers.Authorization)
+    vim.env.VIMPACK_TEST_TOKEN = nil
+  end)
+
+  it('propagates an auth failure instead of downloading unauthenticated', function()
+    vim.env.VIMPACK_TEST_TOKEN = nil
+    local resolved, err = registry.resolve(
+      { name = 'x', version = '1', url = 'https://h/x.zip' },
+      { auth = { env = 'VIMPACK_TEST_TOKEN' } }
+    )
+    assert_nil(resolved)
+    assert_match('VIMPACK_TEST_TOKEN', err)
+  end)
 end)
 
 describe('auth headers', function()