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/dap/init.lua

@@ -30,6 +30,7 @@ return {
       -- Load language-specific configs
       require("plugins.dap.rust")
       require("plugins.dap.js")
+      require("plugins.dap.java")
     end,
   },