nvim: fill gaps, add knap.nvim, clean up stubs

- Java DAP: add java-debug-adapter + java-test via mason, populate
  jdtls bundles with debug/test JARs
- neotest: add marilari88/neotest-vitest adapter
- git: promote diffview.nvim to first-class plugin with keymaps
  (<leader>gD diff, <leader>gH file history)
- pandoc: replace stub with knap.nvim for live markdown→PDF preview
  via pandoc + lualatex + zathura (<leader>mp toggle, <leader>mj jump)
- remove 17 empty stub files left over from AstroNvim migration

dot_config/nvim/lua/plugins/git.lua

@@ -41,6 +41,16 @@ return {
     },
   },
 
+  -- Split diff view and file history
+  {
+    "sindrets/diffview.nvim",
+    cmd = { "DiffviewOpen", "DiffviewClose", "DiffviewFileHistory" },
+    keys = {
+      { "<leader>gD", "<cmd>DiffviewOpen<cr>",          desc = "Diff working tree" },
+      { "<leader>gH", "<cmd>DiffviewFileHistory %<cr>", desc = "File history"      },
+    },
+  },
+
   -- Magit-style full git interface
   {
     "NeogitOrg/neogit",