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/pandoc.lua

@@ -1,2 +1,19 @@
--- Removed: not part of the from-scratch config
-return {}
+return {
+  {
+    "frabjous/knap",
+    ft = { "markdown" },
+    keys = {
+      { "<leader>mp", function() require("knap").toggle_autopreviewing() end, ft = "markdown", desc = "Toggle live preview" },
+      { "<leader>mj", function() require("knap").forward_jump() end,          ft = "markdown", desc = "SyncTeX forward jump"  },
+    },
+    config = function()
+      vim.g.knap_settings = {
+        -- markdown → PDF via pandoc
+        mdoutputext          = "pdf",
+        mdtopdf              = "pandoc %docroot% -o %outputfile% --pdf-engine=lualatex",
+        mdtopdfviewerlaunch  = "zathura %outputfile%",
+        mdtopdfviewerrefresh = "none", -- zathura watches the file itself
+      }
+    end,
+  },
+}