Add diffview.nvim for reviewing diffs and file history

Installed via vim.pack, matching how the other plugins here are
managed. Adds a new <leader>g "diff" group: gd opens a Diffview
against the index, gc closes it, gh shows history for the current
file, gH for the branch.

Verified headlessly: config/plugins loads clean, :DiffviewOpen is
registered, and opening/closing a Diffview against this repo works
with no errors.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013t7aQbc4EyYtab1crvSFTd

neovim/nvim-pack-lock.json

@@ -1,5 +1,9 @@
 {
   "plugins": {
+    "diffview.nvim": {
+      "rev": "4516612fe98ff56ae0415a259ff6361a89419b0a",
+      "src": "https://github.com/sindrets/diffview.nvim"
+    },
     "fzf-lua": {
       "rev": "8671012a7b2a1648595952c655039d51fb301c14",
       "src": "https://github.com/ibhagwan/fzf-lua"