Fix vCard folding and group parsing rules

Accept horizontal-tab continuations when unfolding content lines and
allow hyphens in group names, matching the MIME-DIR and vCard grammar.
Also add focused private tests covering both cases.

AI-Assisted: yes
AI-Tool: OpenAI Codex / gpt-5.4 xhigh

src/vcard/private/internals.nim

@@ -163,8 +163,9 @@ proc readGroup*(p: var VCardParser): Option[string] =
   ## name. If there is not a valid group the read position is left unchanged.
 
   p.setBookmark
+  let validChars = ALPHA_NUM + {'-'}
   var ch = p.read
-  while ALPHA_NUM.contains(ch): ch = p.read
+  while validChars.contains(ch): ch = p.read
 
   if (ch == '.'):
     result = some(readSinceBookmark(p)[0..^2])