Retrace ability must be reworked as additional cost, not alternative (example: KickerAbility)

Encountered a bug today with Fires of Invention and Wrenn and Six's emblem. Fires should allow you to cast spells using Wrenn's retrace effect from the graveyard but it didn't, instead prompting me to spend mana. This also happened when attempting to escape Uro, Titan of Nature's Wrath.

Start by reproducing the interaction between Fires of Invention and Wrenn and Six's retrace effect, then compare it with escaping Uro, Titan of Nature's Wrath. Trace the retrace and escape handling to determine where the ability is treated as an alternative rather than an additional cost; done means both scenarios allow the expected graveyard casts without incorrectly requiring mana.

Written by the indexing model from the issue text.