feat(presets): add tectonic preset

Adds a tectonic preset for the modern Rust-based LaTeX engine, which
auto-downloads packages and requires no TeX installation. Reuses
parse_latexmk since tectonic emits the same file:line: message
diagnostic format.

lua/preview/presets.lua

@@ -153,6 +153,22 @@ M.pdflatex = {
   open = true,
 }
 
+---@type preview.ProviderConfig
+M.tectonic = {
+  ft = 'tex',
+  cmd = { 'tectonic' },
+  args = function(ctx)
+    return { ctx.file }
+  end,
+  output = function(ctx)
+    return (ctx.file:gsub('%.tex$', '.pdf'))
+  end,
+  error_parser = function(output)
+    return parse_latexmk(output)
+  end,
+  open = true,
+}
+
 ---@type preview.ProviderConfig
 M.markdown = {
   ft = 'markdown',