Fix GNU3 demangler template and backref handling

Update the GNU3 parser to use shared DemangledTypeNode references for substitutions, template substitutions, and nested type/name construction so backrefs preserve structure instead of copying stale formatted strings.

Fix template argument parsing for non-type template parameter declarations, expression arguments, argument packs, and generic lambda auto parameters while preserving enclosing template substitution state.

Carry platform context through GNU3 demangling/finalization so rendered and finalized types use the same delayed DemangledTypeNode representation as the other demangler paths.

Author: plafosse