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/html_renderer.rs

@@ -77,6 +77,27 @@ fn centered_heading_gets_centered_class() {
     assert!(out.contains("<h1 class=\"centered\" id=\"T\">T</h1>"));
 }
 
+#[test]
+fn spaceless_heading_renders_as_heading() {
+    // `==Heading==` (no surrounding spaces) is a heading, like vimwiki — not a
+    // literal `<p>==Heading==</p>`.
+    let out = render("==Plain==\n");
+    assert!(
+        out.contains(
+            "<div id=\"Plain\"><h2 id=\"Plain\" class=\"header\">\
+             <a href=\"#Plain\">Plain</a></h2></div>"
+        ),
+        "got: {out}"
+    );
+    // The spaceless + keyword combo from the report: a header, not a badge.
+    let out2 = render("==TODO==\n");
+    assert!(
+        out2.contains("<h2 id=\"TODO\" class=\"header\">"),
+        "got: {out2}"
+    );
+    assert!(!out2.contains("<p>"), "not a paragraph: {out2}");
+}
+
 #[test]
 fn heading_keyword_renders_as_plain_text_not_badge() {
     // `== TODO ==` is a section title, not an inline keyword: it must render as