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>

README.md

@@ -234,6 +234,7 @@ require('nuwiki').setup({
       exclude_files    = {},             -- glob patterns
       -- Checkbox progression characters: " .oOX" by default.
       listsyms         = ' .oOX',
+      listsym_rejected = '-',           -- glyph for a cancelled checkbox [-]
       listsyms_propagate = true,
     },
   },
@@ -327,6 +328,7 @@ Each is a boolean. All default to `true` except `mouse`.
 | `diary_sort` | string | `'desc'` | `'desc'` \| `'asc'` |
 | `diary_header` | string | `'Diary'` | any heading text |
 | `listsyms` | string | `' .oOX'` | checkbox progression chars |
+| `listsym_rejected` | string | `'-'` | glyph for a cancelled checkbox (`[-]`); first char used |
 | `listsyms_propagate` | bool | `true` | `true` \| `false` |
 | `list_margin` | int | `-1` | `-1` (auto) or `≥ 0` |
 | `links_space_char` | string | `' '` | char that replaces spaces in synthesised link paths |