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/28_latex.lua

@@ -0,0 +1,43 @@
+-- vimtex integration for raw .tex workflows (paper drafts, AEA submissions,
+-- beamer slides). Rides in the `markdown` cat because Quarto users routinely
+-- also write standalone .tex.
+
+local Config = require('config')
+local later = MiniDeps.later
+local nix = require('config.nix')
+
+later(function()
+  if not nix.get_cat("markdown", false) then
+    return
+  end
+
+  Config.add("vimtex")
+
+  local ok, vimtex = pcall(require, "vimtex")
+  if not ok then
+    vim.notify("vimtex not available", vim.log.levels.WARN)
+    return
+  end
+
+  -- Keep conservative defaults: latexmk continuous compilation off, single
+  -- viewer (zathura falls back to Evince on most setups).
+  vimtex.setup({
+    enabled = true,
+    compile_on_save = false,
+    compiler = "latexmk",
+    -- Avoid hooking spell/formatting into our global <leader> group;
+    -- vimtex stashes its own <localleader> mappings automatically.
+  })
+
+  -- Filetype detection is normally on, but force it explicitly so .tex files
+  -- opened outside Quarto still pick up the LSP + viewer hooks.
+  vim.api.nvim_create_autocmd("BufReadPost", {
+    pattern = { "*.tex", "*.sty", "*.cls" },
+    callback = function(args)
+      local buf = args.buf
+      if vim.api.nvim_buf_is_loaded(buf) then
+        vim.bo[buf].filetype = "tex"
+      end
+    end,
+  })
+end)