fix(parser): accept spaceless headings (==Heading==) like vimwiki (#7)

The heading lexer required a space after the opening `=`s, so `==Heading==`
fell through to a literal `<p>==Heading==</p>` instead of an `<h2>`. vimwiki
accepts the spaceless form, so a migrated page whose top heading was
`==TODO==` (or `====Progress====`) rendered with no heading at all.

Drop the space-after-marker requirement; the existing title trim already
handles one optional space per side, so both `== H ==` and `==H==` work.
Marker-only (`======`) and unbalanced (`==a==b`, `==> x`) lines still stay
paragraphs (title-present + matching trailing-`=` checks). Regression tests
added at the lexer and renderer levels.

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

crates/nuwiki-core/tests/lexer.rs

@@ -85,6 +85,32 @@ fn centered_heading_is_marked_centered() {
     assert!(matches!(lexed[2], HeadingClose));
 }
 
+#[test]
+fn spaceless_heading_is_a_heading() {
+    // vimwiki accepts `==Heading==` with no spaces around the text.
+    let lexed = lex("==Plain==\n");
+    assert_eq!(
+        lexed,
+        vec![
+            HeadingOpen {
+                level: 2,
+                centered: false
+            },
+            text("Plain"),
+            HeadingClose,
+            Newline,
+        ]
+    );
+}
+
+#[test]
+fn marker_only_or_unbalanced_lines_are_not_headings() {
+    // No title between markers, and trailing `=` not at line end → plain text.
+    assert!(!matches!(lex("======\n")[0], HeadingOpen { .. }));
+    assert!(!matches!(lex("==> arrow\n")[0], HeadingOpen { .. }));
+    assert!(!matches!(lex("==a==b\n")[0], HeadingOpen { .. }));
+}
+
 #[test]
 fn heading_with_inline_bold() {
     let lexed = lex("== Hello *world* ==\n");