ManaPaidSourceWatcher tracks how many treasures were spent, not the mana generated by treasures

All currently existing "do thing based off being paid for by treasures" that I can find care about the amount of mana generated by treasures (or that any mana from treasures was spent on it), rather than the number of actual treasures used. The watcher needs to be redone such that the event includes the amount of mana if not currently available, and then it increments the `treasure` counter by the amount of mana generated by treasures.

Start by locating ManaPaidSourceWatcher and the event it consumes. Trace how treasure payments are reported and how the watcher updates its counter; done means the event exposes the generated mana when needed and the counter reflects mana generated by treasures rather than the number of treasures used.

Written by the indexing model from the issue text.