feat(config): make listsym_rejected configurable (P2)

The rejected/cancelled checkbox glyph was a hardcoded `const REJECTED = '-'`.
ListSyms now carries a `rejected` field (new_with_rejected); a per-wiki
`listsym_rejected` key (default `-`) is added to WikiConfig, and
WikiConfig::list_syms() builds the palette with it. Routed the three
config-driven ListSyms construction sites (parse path in lib.rs + the
toggle/cycle commands) through list_syms(), so a custom rejected glyph both
lexes and round-trips.

Tests: listsyms::custom_rejected_glyph_is_honoured + config round-trip.
README/doc/lua config comment updated. fmt clean; 0 Rust test failures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lua/nuwiki/config.lua

@@ -26,7 +26,7 @@ M.defaults = {
   --   html_path, template_path, template_default, template_ext,
   --   template_date_format, css_name, auto_export, auto_toc,
   --   html_filename_parameterization, exclude_files, color_dic
-  --   listsyms, listsyms_propagate, list_margin,
+  --   listsyms, listsym_rejected, listsyms_propagate, list_margin,
   --   links_space_char
   wikis = nil,