Fix manually sleeping persistent-island bodies

[zdtango]

Summary

• mark explicit sleep transitions as body changes
• move manually slept bodies into sleeping islands
• treat their stale contacts as sleep-traversal boundaries until narrow phase updates

Reproduction

Calling RigidBody::sleep on an active body in a large persistent island panicked in island sleep traversal.

Validation

• cargo test -p rapier3d --lib
• reproduced and fixed in a 2,001-body Rapier.js world