fix: fall back to /tmp for buffers without a backing file

Problem: markdown and gfm presets fail when the buffer has no file on
disk (e.g. unnamed buffer with `ft=markdown`, or a named buffer whose
path doesn't exist yet) because `build_context` passes a nonexistent
path to pandoc and `compile` guards reject empty buffer names.

Solution: `build_context` now detects missing files and redirects
`ctx.file` to `/tmp/{bufnr}-{name}`. `compile` writes buffer contents
to that temp path via `vim.fn.writefile` instead of `:silent! update`.

lua/preview/compiler.lua

@@ -215,8 +215,14 @@ function M.compile(bufnr, name, provider, ctx, opts)
     return
   end
 
-  if vim.bo[bufnr].modified then
-    vim.cmd('silent! update')
+  local buf_file = vim.api.nvim_buf_get_name(bufnr)
+  if buf_file ~= '' and buf_file == ctx.file then
+    if vim.bo[bufnr].modified then
+      vim.cmd('silent! update')
+    end
+  else
+    local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+    vim.fn.writefile(lines, ctx.file)
   end
 
   local s = get_state(bufnr)