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/install_spec.lua

@@ -126,6 +126,33 @@ describe('install end to end', function()
     assert_match(missing, results[1].err)
   end)
 
+  it('installs with the cache on a different filesystem from the pack root', function()
+    local root = tmpdir()
+    local pack_root = vim.fs.joinpath(root, 'pack')
+    local other = otherfs_dir(root)
+    if not other then
+      return
+    end
+
+    local real_cache_dir = fsmod.cache_dir
+    fsmod.cache_dir = function()
+      return vim.fs.joinpath(other, 'vimpack')
+    end
+
+    local archive = make_tarball(root, '1.0.0', 'return 1')
+    local cfg = assert(config.normalize({
+      plugins = { { name = 'demo.nvim', version = '1.0.0', url = 'file://' .. archive } },
+    }))
+    local data = manifest.empty()
+    local ok, results = pcall(install, cfg, data, pack_root)
+    fsmod.cache_dir = real_cache_dir
+    fsmod.rm_rf(other)
+
+    assert_true(ok, tostring(results))
+    assert_true(results[1].ok, tostring(results[1].err))
+    assert_eq({ 'return 1' }, vim.fn.readfile(vim.fs.joinpath(pack_root, 'start', 'demo.nvim', 'lua', 'demo.lua')))
+  end)
+
   it('moves a plugin from start to opt', function()
     local root = tmpdir()
     local pack_root = vim.fs.joinpath(root, 'pack')