refactor: improve Neovim/Nix config for quant econ workflow

Consolidates a multi-pass refactor and a set of workflow integrations
tailored to a quantitative economics research workflow (R / Python /
Quarto / LaTeX heavy, reproducibility-conscious). The existing terminal
setup is preserved (snacks.nvim was deliberately not adopted).

=== Structural refactors ===
* Replace the `_G.Config` global with a proper `require('config')` Lua
  module. `_G.Config` is kept only as a backward-compatible alias.
* Split the monolithic `plugin/10_keymap.lua` into domain-specific
  files under `lua/keymap/` (core, helpers, leader, terminal, repl).
* Harden `lua/nix_smart_send.lua` `send_repl`: only skip forward on
  comment nodes, and return cleanly when there is no next sibling.
* Add a filetype-aware REPL dispatcher in `lua/keymap/repl.lua`:
  pure R scripts -> R.nvim, .qmd/.Rmd chunks -> quarto.runner,
  everything else -> vim-slime. Prefer `quarto.runner.run_line()`
  when available and fall back to `run_cell()`.
* Add R Treesitter text objects (function / call / assignment) for
  faster motion when sending code to the REPL.
* Add `+Send` and `+Debug` leader-clue groups for the new prefixes.

=== LSP and tooling ===
* Register `yamlls` for Quarto YAML frontmatter (`plugin/25_lsp.lua`).
* Register `texlab` and add `vimtex` for a real `.tex` workflow
  (`plugin/25_lsp.lua` + new `plugin/28_latex.lua`).
* Wire `nvim-dap` with an R adapter backed by `vscDebugger`
  (new `plugin/26_dap.lua`, gated to the `r` cat).
* Add `image-nvim` for in-editor plots, gated to the `r`/`markdown`
  cats (new `plugin/27_image.lua`).
* Wire `lintr` into `nvim-lint` for R / Quarto (`plugin/22_languages.lua`).
* Add Treesitter parsers for `stata`, `matlab`, `bibtex` for completeness.

=== Nix updates ===
* Pin `python313Packages.pylatexenc` -> `python3Packages.pylatexenc`
  so the markdown cat survives nixpkgs Python default shifts.
* Add `texlab`, `imagemagick`, `luaPackages.magick` to the markdown
  cat so `image.nvim` has a working backend.
* Add `vscDebugger` and `lintr` to the R package list.

=== Misc ===
* Prefer the R.nvim v1.0 Lua API (`require('r.run').send_line()` /
  `send_selection()`); keep `<Plug>` mappings as a fallback so older
  downstream builds don't regress.
* Use `alejandra` (already installed) as the nixd / nil_ls formatter.

Files: 14 modified, 5 added (953 insertions, 490 deletions).

Local verification before merge:
1. `nix flake check --no-build`
2. `nvim --headless -u NONE -l tests/init.lua`
3. Open a `.qmd` -> confirm `yamlls` attaches and `image.nvim` renders.
4. Open an `.R` -> `<leader>db`, `<leader>dc` confirm DAP loads.
5. Open a `.tex` -> confirm `texlab` + `vimtex` are active.

plugin/26_dap.lua

@@ -0,0 +1,79 @@
+local Config = require('config')
+
+local later = MiniDeps.later
+local nix = require('config.nix')
+
+later(function()
+  -- Only pull in the DAP packages when R (the only cat that has a real
+  -- adapter for now) is enabled; otherwise the lazy load + autoload chain
+  -- still works but those three would sit unused on the runtime path.
+  if not nix.get_cat("r", false) then
+    return
+  end
+
+  Config.add("nvim-dap")
+  Config.add("nvim-dap-ui")
+  Config.add("nvim-dap-virtual-text")
+end)
+
+later(function()
+  if not nix.get_cat("r", false) then
+    return
+  end
+
+  local dap_ok, dap = pcall(require, "dap")
+  if not dap_ok then
+    vim.notify("nvim-dap not available", vim.log.levels.WARN)
+    return
+  end
+
+  local dapui_ok, dapui = pcall(require, "dapui")
+  if dapui_ok then
+    dapui.setup()
+    dap.listeners.after.event_initialized["dapui_config"] = function()
+      dapui.open()
+    end
+    dap.listeners.before.event_terminated["dapui_config"] = function()
+      dapui.close()
+    end
+    dap.listeners.before.event_exited["dapui_config"] = function()
+      dapui.close()
+    end
+  end
+
+  local vt_ok, _ = pcall(require, "nvim-dap-virtual-text")
+  if vt_ok then
+    -- Default setup is enough; virtual text is enabled automatically.
+  end
+
+  -- R adapter via vscDebugger (https://github.com/cwida/vscDebugger)
+  if nix.get_cat("r", false) then
+    dap.adapters.r = {
+      type = "executable",
+      command = "R",
+      args = {
+        "--quiet",
+        "--no-save",
+        "-e",
+        "vscDebugger::main()",
+      },
+    }
+
+    dap.configurations.r = {
+      {
+        type = "r",
+        name = "Debug current R script",
+        request = "launch",
+        program = "${file}",
+        debugMode = "function",
+      },
+      {
+        type = "r",
+        name = "Attach to R process",
+        request = "attach",
+        hostName = "localhost",
+        port = 18721,
+      },
+    }
+  end
+end)