Add Eusine rematch and rework event handling in Eusine

[vulcandth]

See the reworked flow here:

Mermaid Diagram

Note: Have not tested in game yet.

[Copilot]

Pull request overview

This PR adds a post–Ho-Oh rematch for Eusine and reworks how Eusine’s quest progression is tracked across Tin Tower and Eusine’s House by introducing new event flags and adjusting respawn behavior.

Changes:

• Add a new quest-completion flag (EVENT_EUSINE_SAW_HO_OH) and update Tin Tower / Eusine’s House scripts to use it.
• Add a new rematch trainer entry (EUSINE_2) and a new “beaten” event flag (EVENT_BEAT_EUSINE_2) to gate the rematch.
• Adjust Ho-Oh respawn handling to no longer reset Eusine’s house presence flag.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
maps/TinTower1F.asm	Gates Eusine’s post–Ho-Oh appearance and sets new quest-completion flags after the post-fight scene.
maps/EusinesHouse.asm	Adds post–Ho-Oh dialogue + optional rematch flow; reworks grandpa’s reward logic around the new quest flag.
engine/events/specials.asm	Removes resetting Eusine’s house event during Ho-Oh respawn logic.
data/trainers/parties.asm	Adds Eusine’s rematch trainer party definition.
constants/trainer_constants.asm	Adds the EUSINE_2 trainer constant.
constants/event_flags.asm	Adds EVENT_BEAT_EUSINE_2 and EVENT_EUSINE_SAW_HO_OH.

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