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/lsp.lua

@@ -49,6 +49,8 @@ return {
         -- DAP
         "codelldb",
         "js-debug-adapter",
+        "java-debug-adapter",
+        "java-test",
       },
       auto_update = false,
       run_on_start = true,