[matthew-walters]

Fixes #1049.

When an awake dynamic body was changed to Fixed, it was removed from its persistent island but remained in the active body set indefinitely.

This routes the type change through the existing full island-removal path, which removes both persistent and active membership and repairs indices affected by swap-removal.

The regression verifies that:

• fixed bodies leave both island representations
• the surviving active-body entry remains valid
• bodies changed back to dynamic are registered exactly once and respond to gravity.

Validation:

• cargo test -p rapier3d --test persistent_islands
• cargo test -p rapier2d --lib
• cargo fmt --all -- --check
• RUSTFLAGS="-D warnings" cargo clippy -p rapier2d -p rapier3d
• git diff --check