Erase client lifecycle handlers through avatar wrapper

[ogyrec-o]

Summary

Follow-up to #124 for #123.

Wraps concrete ClientLifecycleHandler values in an avatar-owned erased registration payload before passing them through the hidden freven_world_api backend boundary.

Why

#124 added the SDK-facing registration boundary, but the initial hidden plumbing passed the concrete handler type H through Box<dyn Any + Send + Sync>.

The engine runtime does not know the concrete H, so it cannot downcast and recover a callable ClientLifecycleHandler.

This PR adds ClientLifecycleHandlerRegistration, which stores Box<dyn ClientLifecycleHandler>. The runtime can downcast the hidden Any payload to this stable avatar-owned wrapper and then store/call the boxed handler.

Boundaries

• SDK API plumbing only.
• No engine runtime implementation yet.
• No Vanilla behavior changes.
• No server authority changes.
• No client prediction as authority.
• Preserves the world/avatar dependency boundary.

Validation

• cargo +stable check --locked -p freven_avatar_api
• cargo +stable test --locked --workspace
• cargo +stable clippy --locked --workspace --all-targets -- -D warnings
• cargo +stable fmt --all -- --check
• git --no-pager diff --check

Related

• Add client lifecycle local state boundary for native/guest client systems #123
• Add client lifecycle handler registration boundary #124
• Add bounded client terrain intent aggregation for near-frame place-break cancel freven-vanilla#94