fix(export): emit one CSS class per keyword for independent colouring

All keywords previously shared `class="todo"`, so a stylesheet couldn't
distinguish e.g. red TODO from green DONE. Emit a distinct class per keyword
(todo/done/started/fixme/fixed/xxx); TODO keeps the vimwiki `todo` class so
stock vimwiki CSS still styles it.

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

crates/nuwiki-core/tests/html_renderer.rs

@@ -196,8 +196,20 @@ fn inline_code_is_escaped() {
 
 #[test]
 fn keyword_spans() {
-    let out = render("TODO write tests\n");
-    assert!(out.contains("<span class=\"keyword keyword-todo\">TODO</span>"));
+    // One class per keyword so a stylesheet can colour groups differently.
+    // TODO keeps the vimwiki `todo` class.
+    let cases = [
+        ("TODO x\n", "<span class=\"todo\">TODO</span>"),
+        ("DONE x\n", "<span class=\"done\">DONE</span>"),
+        ("STARTED x\n", "<span class=\"started\">STARTED</span>"),
+        ("FIXME x\n", "<span class=\"fixme\">FIXME</span>"),
+        ("FIXED x\n", "<span class=\"fixed\">FIXED</span>"),
+        ("XXX x\n", "<span class=\"xxx\">XXX</span>"),
+    ];
+    for (src, expected) in cases {
+        let out = render(src);
+        assert!(out.contains(expected), "src {src:?} -> {out}");
+    }
 }
 
 #[test]