fix(rapier): reject snapshot restore after body membership changes

[Ram-Dawson]

Fixes #298

Problem

Native Rapier snapshots can restore the native world, but do not recreate Dart pose targets or application-owned body registrations. restore() could accept a snapshot after a body was created or destroyed, leaving the native world and Dart body registry out of sync.

Change

Encode the Dart body-handle set alongside the native snapshot bytes. Decode and compare that set with the current _bodies keys before calling native restore. Return false when membership differs, leaving the current native world untouched.

Validation

• From packages/flutter_scene_rapier: fvm flutter test test/world_snapshot_test.dart
• The focused test covers a body created after snapshot and a body destroyed after snapshot; both reject restore and continue simulation safely.
• The companion reproduction repository provides the failing baseline and fixed tag for the same Android interaction.

Scope and limitation

This PR protects the low-level RapierWorld contract. It does not implement application-level object-graph rollback, automatically destroy Dart-owned objects, reconstruct external pose targets, or change snapshot behavior when body membership is unchanged.