Add client lifecycle handler registration boundary

[ogyrec-o]

Summary

Adds the SDK-facing registration boundary for stateful client lifecycle handlers.

This introduces:

• ClientLifecycleHandler
• AvatarLifecycleRegistrationExt::register_client_lifecycle_handler
• hidden ModContext / ModContextBackend plumbing for runtime implementations

This is the SDK half of #123 and is needed before implementing bounded Vanilla client terrain intent aggregation in frevenengine/freven-vanilla#94.

Why

Vanilla block interaction currently uses stateless function hooks:

• on_start_client(fn)
• on_tick_client(fn)

That is not sufficient for small deterministic per-client-session state such as a bounded near-frame terrain intent cancel window. Using process-global state in Vanilla would be the wrong long-term boundary.

This PR adds a typed SDK registration surface so the runtime can own one mutable client lifecycle handler per mod/client session.

Boundaries

• SDK API/registration boundary only.
• No runtime implementation in this PR.
• No Vanilla behavior changes.
• No server authority changes.
• No client prediction as authority.
• No protocol changes.
• No process-global state workaround.
• Runtime implementation is expected in frevenengine/freven-engine after this lands.

Validation

• cargo +stable check --locked -p freven_avatar_api
• cargo +stable check --locked -p freven_world_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 bounded client terrain intent aggregation for near-frame place-break cancel freven-vanilla#94
• frevenengine/freven-engine#400
• Coalesce net-neutral same-frame place break intents freven-vanilla#93
• frevenengine/freven-boot#143