Add : + continuation marker for multiple dd elements in definition lists

- Maintain spec compliance: blank lines within definition content create
  paragraphs in the same <dd> element
- Add new `: +` marker syntax to explicitly create additional <dd> elements
  for the same term when needed
- Update HtmlToDjot converter to output `: +` markers for multiple <dd> elements
- Add skip mechanism for intentional spec deviations in OfficialTestSuiteTest
  (currently empty as we're fully spec compliant)

This approach gives users explicit control over when to create multiple
definitions vs multiple paragraphs within one definition, while maintaining
backwards compatibility with the djot spec.

Example:
```
: Term

  First paragraph (same dd)

  Second paragraph (same dd)

: +

  Third paragraph (new dd)
```

Author: dereuromark

tests/OfficialTestSuiteTest.php

@@ -28,6 +28,17 @@
 #[Group('official')]
 class OfficialTestSuiteTest extends TestCase
 {
+    /**
+     * Tests to skip due to intentional spec deviations
+     *
+     * Format: 'filename_index' => 'reason for deviation'
+     *
+     * @var array<string, string>
+     */
+    protected const INTENTIONAL_DEVIATIONS = [
+        // Currently no intentional deviations
+    ];
+
     protected DjotConverter $converter;
 
     protected function setUp(): void
@@ -135,6 +146,12 @@ public static function officialTestProvider(): array
     #[DataProvider('officialTestProvider')]
     public function testOfficialSuite(string $input, string $expected, string $file, int $index): void
     {
+        $testName = basename($file, '.test') . '_' . $index;
+
+        if (isset(self::INTENTIONAL_DEVIATIONS[$testName])) {
+            $this->markTestSkipped('Intentional deviation: ' . self::INTENTIONAL_DEVIATIONS[$testName]);
+        }
+
         $result = $this->converter->convert($input);
 
         // Normalize whitespace for comparison

tests/TestCase/Parser/BlockParserTest.php

@@ -204,6 +204,50 @@ public function testParseDefinitionListDdAttributeBeforeContentNotParsed(): void
         $this->assertNull($dd->getAttribute('class'));
     }
 
+    public function testParseDefinitionListBlankLineKeepsSameDd(): void
+    {
+        // Blank lines within definition content create paragraphs in same dd (spec behavior)
+        $djot = ": Term\n\n  First paragraph\n\n  Second paragraph";
+        $doc = $this->parser->parse($djot);
+
+        $dl = $doc->getChildren()[0];
+        $this->assertInstanceOf(DefinitionList::class, $dl);
+
+        // Should have: dt + dd = 2 children (one dd with multiple paragraphs)
+        $children = $dl->getChildren();
+        $dds = array_filter($children, fn ($c) => $c->getType() === 'definition_description');
+        $this->assertCount(1, $dds);
+    }
+
+    public function testParseDefinitionListContinuationMarker(): void
+    {
+        // `: +` marker creates additional dd for same term
+        $djot = ": Term\n\n  First definition\n\n: +\n\n  Second definition";
+        $doc = $this->parser->parse($djot);
+
+        $dl = $doc->getChildren()[0];
+        $this->assertInstanceOf(DefinitionList::class, $dl);
+
+        // Should have: dt + dd + dd = 3 children
+        $children = $dl->getChildren();
+        $dds = array_filter($children, fn ($c) => $c->getType() === 'definition_description');
+        $this->assertCount(2, $dds);
+    }
+
+    public function testParseDefinitionListEmptyDd(): void
+    {
+        // Term with no definition content should still create empty dd
+        $djot = ': Term';
+        $doc = $this->parser->parse($djot);
+
+        $dl = $doc->getChildren()[0];
+        $this->assertInstanceOf(DefinitionList::class, $dl);
+
+        $children = $dl->getChildren();
+        $dds = array_filter($children, fn ($c) => $c->getType() === 'definition_description');
+        $this->assertCount(1, $dds);
+    }
+
     public function testParseThematicBreak(): void
     {
         $doc = $this->parser->parse('---');