feat(core): make checkbox lexing honor a configurable listsyms palette

Add a ListSyms type (vimwiki's g:vimwiki_listsyms) and thread it through
the lexer so checkbox glyphs are recognized from the configured palette
rather than the hardcoded ' .oOX'. Glyphs map to the existing five-bucket
CheckboxState, so the HTML renderer and AST consumers are unchanged.
VimwikiSyntax::parse_with_listsyms opts into a custom palette; the trait
parse() keeps the default.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/nuwiki-core/tests/lexer.rs

@@ -309,6 +309,45 @@ fn checkbox_states() {
     }
 }
 
+#[test]
+fn checkbox_states_honor_custom_palette() {
+    use nuwiki_core::listsyms::ListSyms;
+
+    let lex_with = |text: &str, syms: ListSyms| -> Vec<VimwikiTokenKind> {
+        VimwikiLexer::new()
+            .with_listsyms(syms)
+            .lex(text)
+            .into_iter()
+            .map(|t| t.kind)
+            .collect()
+    };
+
+    // 3-glyph palette ` x✓`: empty / half / done, plus the fixed rejected `-`.
+    let syms = ListSyms::new(" x✓");
+    let cases: &[(&str, CheckboxState)] = &[
+        ("- [ ] a\n", CheckboxState::Empty),
+        ("- [x] a\n", CheckboxState::Half),
+        ("- [✓] a\n", CheckboxState::Done),
+        ("- [-] a\n", CheckboxState::Rejected),
+    ];
+    for (line, expected) in cases {
+        let lexed = lex_with(line, syms.clone());
+        assert!(
+            matches!(lexed[1], Checkbox(s) if s == *expected),
+            "input {line:?} expected {expected:?}, got {:?}",
+            lexed[1]
+        );
+    }
+
+    // Default-palette glyphs are not checkboxes under this palette: `[o]`
+    // is plain inline text, so no Checkbox token is emitted.
+    let lexed = lex_with("- [o] a\n", syms);
+    assert!(
+        !lexed.iter().any(|k| matches!(k, Checkbox(_))),
+        "expected no checkbox token, got {lexed:?}"
+    );
+}
+
 // ===== Blockquotes =====
 
 #[test]