fix(html): keyword-in-heading badge + brackets in wikilink descriptions

Two export bugs reported against v0.4.0:

1. A heading whose text is a keyword (`== TODO ==`) rendered as an inline
   `<span class="todo">` badge with an *empty* id, instead of a styled
   header. Root causes: `inline_text` dropped Keyword nodes (so the heading
   id/anchor came out empty, or `My TODO list` → `My  list`), and the
   keyword span was emitted inside the heading. Fix: `inline_text` now
   includes the keyword's literal text (via new `Keyword::label()`), and
   heading content renders keywords as plain text (flatten_keywords) so a
   keyword title looks like a header. Keyword badges still render in body
   text as before.

2. A `]` inside a wikilink description (e.g. `[[page|Task [B-1]]]`) closed
   the link at the first `]]`, truncating the description and leaking a
   stray `]`. The close-scan is now bracket-aware (find_wikilink_close):
   inner `[ ]` pairs are balanced so the link closes at the correct `]]`.

Added regression tests for both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/nuwiki-core/src/syntax/vimwiki/lexer.rs

@@ -875,8 +875,10 @@ impl<'src> LexState<'src> {
                 flush(self, &mut buf, &mut buf_start, abs_col);
                 self.emit(VimwikiTokenKind::WikiLinkOpen, abs_col, abs_col + 2);
                 i += 2;
-                // Lex until `]]` (absolute index into `slice`).
-                let close_abs_in_slice = after_open.find("]]").map(|c| i + c);
+                // Lex until the closing `]]`. A single `]` inside the body —
+                // e.g. a `[TICKET-1]` in the description — is literal text, so
+                // we balance inner `[ ]` rather than stopping at the first `]]`.
+                let close_abs_in_slice = find_wikilink_close(after_open).map(|c| i + c);
                 let inner_end = close_abs_in_slice.unwrap_or(slice.len());
                 self.lex_link_body(slice, i, line_col_start, inner_end, true);
                 i = inner_end;
@@ -1062,6 +1064,27 @@ impl<'src> LexState<'src> {
 /// Parse the bit after `{{{` on a fence line. Format is forgiving:
 /// `lang` (single word) followed by optional `key=value` pairs separated
 /// by spaces.
+/// Byte offset of the closing `]]` for a wikilink body that begins at the start
+/// of `s` (just after the opening `[[`). Inner `[ ]` pairs are balanced so a
+/// bracketed description like `[[page|Task [B-1]]]` keeps the `[B-1]` and closes
+/// at the final `]]`. A stray single `]` (no matching `[`) is treated as literal
+/// body text. Returns `None` if no closing `]]` is found.
+fn find_wikilink_close(s: &str) -> Option<usize> {
+    let b = s.as_bytes();
+    let mut depth: u32 = 0;
+    let mut i = 0;
+    while i < b.len() {
+        match b[i] {
+            b'[' => depth += 1,
+            b']' if depth > 0 => depth -= 1, // closes an inner '['
+            b']' if i + 1 < b.len() && b[i + 1] == b']' => return Some(i),
+            _ => {}
+        }
+        i += 1;
+    }
+    None
+}
+
 fn parse_fence_attrs(s: &str) -> (Option<String>, HashMap<String, String>) {
     let mut attrs = HashMap::new();
     let mut language: Option<String> = None;