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>

development/vimwiki-gap.md

@@ -93,8 +93,14 @@ fix site.
 - [ ] **On-save autoregen family** — `auto_diary_index`, `auto_generate_links`,
   `auto_generate_tags`, `auto_tags` (only `auto_toc` / `auto_export` exist).
   _Fix:_ `config.rs` `RawWiki`/`WikiConfig` + the `didSave` path.
-- [ ] **`listsym_rejected`** — rejected glyph `-` is hardcoded.
-  _Fix:_ `config.rs` `WikiConfig`.
+- [x] **`listsym_rejected`** — the rejected glyph was a hardcoded `const '-'`.
+  _Fix:_ `ListSyms` gained a `rejected` field + `new_with_rejected()`
+  (`crates/nuwiki-core/src/listsyms.rs`); per-wiki `listsym_rejected` key (default
+  `-`) added to `WikiConfig`; `WikiConfig::list_syms()` builds the palette with
+  it, used by the parse path (`lib.rs`) and the toggle/cycle commands — so a
+  custom rejected glyph both lexes and round-trips. Tests:
+  `listsyms::custom_rejected_glyph_is_honoured` + config round-trip in
+  `index_and_config.rs`.
 - [ ] **`custom_wiki2html` (+`_args`)** and **`base_url`** — external HTML
   converter hook + export URL prefix. _Fix:_ `config.rs` `HtmlConfig`.
 - [ ] **`map_prefix`** — `<Leader>w` hardcoded across the map layer.