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>

doc/nuwiki.txt

@@ -198,6 +198,7 @@ match upstream vimwiki.
 `auto_toc`               `false` — refresh TOC on save.
 `exclude_files`          List of glob patterns excluded from export.
 `listsyms`               `' .oOX'` — checkbox progression characters.
+`listsym_rejected`       `'-'` — glyph for a cancelled checkbox `[-]`.
 `listsyms_propagate`     `true`
 `links_space_char`       `' '`