feat(home): add pandoc with texlive for PDF output

texlive scheme is the `local.texlive.package` option (default scheme-medium)
so consumers can swap schemes without editing the module; TEXMF* vars keep
its cache/config out of $HOME. pandoc PDF defaults to xelatex — pdflatex
fails on combining Unicode (e.g. U+0308 in decomposed ä).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/texlive.nix

@@ -0,0 +1,32 @@
+# TeX Live for pandoc PDF output: pandoc shells out to a LaTeX engine, so PDF
+# conversion needs a TeX distribution (other formats — html, docx, md — do not).
+# The package set is an option so a consumer can swap the scheme without editing
+# this module: scheme-medium covers pandoc's default LaTeX template; override
+# for a larger set (missing .sty files) or a smaller one.
+#
+# TeX Live's font cache and config trees default to ~/.texlive<year> and ~/texmf
+# (written on the first PDF build). The TEXMF* vars route them to XDG dirs.
+
+{ config, lib, pkgs, ... }:
+
+{
+  options.local.texlive.package = lib.mkOption {
+    type = lib.types.package;
+    default = pkgs.texlive.withPackages (ps: [ ps.scheme-medium ]);
+    defaultText = lib.literalExpression
+      "pkgs.texlive.withPackages (ps: [ ps.scheme-medium ])";
+    description = "TeX Live package set. Override for a larger/smaller scheme.";
+  };
+
+  config = {
+    home.packages = [ config.local.texlive.package ];
+
+    # texmf-var is regenerable font/format cache → cacheHome; the other two hold
+    # user config/data trees.
+    home.sessionVariables = {
+      TEXMFHOME   = "${config.xdg.dataHome}/texmf";
+      TEXMFVAR    = "${config.xdg.cacheHome}/texlive/texmf-var";
+      TEXMFCONFIG = "${config.xdg.configHome}/texlive/texmf-config";
+    };
+  };
+}