Split export state into a read-only merged exported and a host-owned meta_exported switch

[jph00]

Message.exported is now read-only: True when an nbdev export directive is present in either the content comments or the meta nbdev dict, merged with comments winning, matching nbdev's own rule. The new read-write Message.meta_exported covers only the meta entry: assigning True writes meta['nbdev']['export']='true', assigning False removes the entry, and content is never touched.

The two sources have different owners - a #| line is text the user edits directly, the meta entry is the host's switch - so the old setter's silent migration (hoisting a content directive into meta on assignment) is gone along with the ambiguity it papered over.

Breaking: assigning Message.exported now raises. Hosts that toggled export state through it should assign meta_exported instead; code that only read exported is unchanged.