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/03_terminal.lua

@@ -1,3 +1,5 @@
+local Config = require('config')
+
 local M = {}
 
 -- Configuration
@@ -31,11 +33,15 @@ function M.split_and_open_terminal()
   vim.cmd("resize " .. math.floor(vim.fn.winheight(0) * 0.9))
   local term_buf = vim.api.nvim_win_get_buf(vim.api.nvim_get_current_win())
   M.opt_term = term_buf
-  
+
   -- Set buffer-local variables for vim-slime
   local job_id = vim.b[term_buf].terminal_job_id
+  if not job_id then
+    vim.notify("Terminal job id not available", vim.log.levels.WARN)
+    return term_buf
+  end
   vim.b[term_buf].slime_config = { jobid = job_id }
-  
+
   return M.opt_term
 end
 
@@ -44,26 +50,45 @@ function M.open_in_terminal(cmd)
   local command = cmd or ""
   local current_window = vim.api.nvim_get_current_win()
   local code_buf = vim.api.nvim_get_current_buf()
-  
+
+  if not vim.api.nvim_buf_is_valid(code_buf) then
+    vim.notify("Code buffer is not valid", vim.log.levels.ERROR)
+    return
+  end
+
   -- Open terminal and get buffer
   local term_buf = M.split_and_open_terminal()
-  
+
+  if not term_buf or not vim.api.nvim_buf_is_valid(term_buf) then
+    vim.notify("Failed to open terminal buffer", vim.log.levels.ERROR)
+    return
+  end
+
   -- Send command if provided
+  local job_id = vim.b[term_buf].terminal_job_id
   if command ~= "" then
-    -- We can use standard slime sending if needed, or direct chan_send for initialization
-    local job_id = vim.b[term_buf].terminal_job_id
-    if job_id then
-      vim.api.nvim_chan_send(job_id, command .. "\r")
+    if not job_id then
+      vim.notify("Terminal job not ready, cannot send command", vim.log.levels.WARN)
+    else
+      local ok, err = pcall(vim.api.nvim_chan_send, job_id, command .. "\r")
+      if not ok then
+        vim.notify("Failed to send command to terminal: " .. tostring(err), vim.log.levels.ERROR)
+      end
     end
   end
-  
+
   -- Configure slime for the ORIGINAL code buffer to point to this new terminal
   -- This makes "Send to Terminal" work immediately
-  local slime_config = { jobid = vim.b[term_buf].terminal_job_id }
-  
-  -- Fix: Set the variable on the captured code buffer, not the current (terminal) buffer
-  vim.api.nvim_buf_set_var(code_buf, "slime_config", slime_config)
-  
+  if job_id then
+    local slime_config = { jobid = job_id }
+
+    -- Fix: Set the variable on the captured code buffer, not the current (terminal) buffer
+    local ok, err = pcall(vim.api.nvim_buf_set_var, code_buf, "slime_config", slime_config)
+    if not ok then
+      vim.notify("Failed to set slime_config on code buffer: " .. tostring(err), vim.log.levels.ERROR)
+    end
+  end
+
   -- Switch back to code buffer
   vim.api.nvim_set_current_win(current_window)
 end