Link label opener doesn't reset closerChar, so emphasis in it becomes a raw tag #95

Follow-up from the second review of #92. This is cosmetic: the output stays correct but isn't minimal.

Opening a link (or inline code) pushes a label buffer but doesn't reset closerChar in MarkdownRendering.kt. If emphasis opens first inside the label, openInline thinks it directly follows the previous closer, even though [ will sit between them. It then writes a raw tag.

Repro

<b>Price:</b><a href=u><b>5</b></a> renders **Price:**[<strong>5</strong>](u) instead of **Price:**[**5**](u).

Fix

Clear closerChar when a label buffer is pushed.