Ymir/docs/aetheria-cutover.md

Aetheria Cutover Notes

First Slice

Cut over projectile travel and impact discovery before trying to replace every Unity physics use.

1. Spawn a Ymir body for each projectile and relevant target hull/contact shape.
2. Step Ymir on the same fixed tick used by Aetheria gameplay.
3. Render Unity projectile GameObjects from Ymir snapshots.
4. Convert Ymir contacts into Aetheria hit/splash policy.
5. Keep Unity colliders as visual/debug witnesses only during the transition.
6. Delete or neuter collider callbacks once the Ymir path owns hit truth.

Negative Checks

• A projectile with a Unity collider disabled still travels and hits through Ymir.
• A Unity OnCollisionEnter callback cannot apply damage independently.
• Replaying from the same canonical creation order and command stream under the accepted Box3D build produces the same ordered Ymir facts.
• Restarting from a checkpoint matches named numeric state tolerances. It emits no authoritative gameplay contact facts until canonical history has replayed through the checkpoint tick and restored exact contact lifecycle identity.
• Mid-flight visible projectile position matches the Ymir snapshot within a named tolerance.

Adapter Boundary

Unity adapter code may:

• submit spawn/despawn/field/body commands
• receive snapshots and contacts
• render GameObjects from snapshots
• publish diagnostics

Unity adapter code may not:

• alter positions as a hidden repair loop
• apply gameplay damage from Unity collisions
• maintain a parallel physics cache that can override Ymir
• treat scene hierarchy as the source of simulation truth