Expose ros-z-debug subscription updates

[schmidma]

Summary

This PR lets users subscribe to live updates from ros-z-debug subscriptions.

A user can now keep a handle to the latest received data, and also wait for changes on that handle. Updates report new data, status changes with retained diagnostic messages, and missed messages when a receiver falls behind.

Closing a subscription closes the update stream. The last retained data and status stay available through the handle.

Motivation

Debug tools often need to react when subscription data changes. Before this PR, callers could only drain a small internal event buffer. That mixed two different jobs: storing recent state and notifying live consumers.

This PR makes the split clearer:

• use latest() and window() to inspect retained data
• use subscribe_updates() to wait for future changes

Reviewer Notes

The main API change is in crates/ros-z-debug/src/subscription.rs:

• drain_events() is replaced by subscribe_updates()
• update receivers only see future updates
• closed subscriptions return SubscriptionUpdateClosed

The event types live in crates/ros-z-debug/src/event.rs.

The subscription lifecycle also moved into SubscriptionState::spawn. This keeps live update stream closure, receive-task cancellation, and weak-state receive-loop ownership in one place. The receive task now exits cooperatively through the existing CancellationToken instead of storing and aborting an AbortHandle from the builders.

Test Plan

• cargo nextest run -p ros-z-debug
• cargo test --doc -p ros-z-debug
• cargo clippy -p ros-z-debug --all-targets --locked -- -D warnings

[rmburg]

LGTM