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/25_lsp.lua

@@ -1,3 +1,4 @@
+local Config = require('config')
 local now_if_args = Config.now_if_args
 
 if not Config.isNixCats then
@@ -16,6 +17,40 @@ now_if_args(function()
     marksman = {
       filetypes = { "markdown", "markdown_inline", "codecompanion" },
     },
+    nil_ls = {
+      settings = {
+        ["nil"] = {
+          formatting = {
+            command = { "alejandra" },
+          },
+        },
+      },
+    },
+    nixd = {
+      settings = {
+        nixd = {
+          formatting = {
+            command = { "alejandra" },
+          },
+          options = {
+            -- Downstream flakes can override these via lib.mkMerge on the lsp config.
+            nixos = { expr = "(builtins.getFlake \"/etc/nixos\").nixosConfigurations.\"default\".options" },
+            home_manager = { expr = "(builtins.getFlake \"/etc/nixos\").homeConfigurations.\"default\".options" },
+          },
+        },
+      },
+    },
+    yamlls = {
+      settings = {
+        yaml = {
+          schemas = {
+            ["https://raw.githubusercontent.com/quarto-dev/quarto-cli/main/src/resources/schema/project.json"] = "**/_quarto.yml",
+            ["https://raw.githubusercontent.com/quarto-dev/quarto-cli/main/src/resources/schema/document-quarto.json"] = "**/*.qmd",
+          },
+        },
+      },
+    },
+    texlab = {},
     julials = {
       settings = {
         julia = {