feat(core): markers and reorder 🙀

- reorders don't interact with markers, but must transit them
- add updated tests
- simplify reorder_group::apply(): the 'common prefix' discussion
was out-of-date as std::mismatch is handled by the ldml_processor
and the core context.
- add_back_markers() promoted to SPI so it can be called from reordering.

Fixes: #10516