[Spec coverage] group/update: playback_state, group_id, group_name verification

[balloob]

Gap

Servers currently emit group/update during player scenarios (adapters/sendspin-go/server/main.go:434-439, :497-502) but:

• No client summary includes the received playback_state, group_id, or group_name.
• runner.py has no comparator for these fields.
• The spec's delta-merge / null-clearing semantics are not exercised.

Proposed coverage

1. Record received group/update fields in the client summary and assert that the server's declared values (e.g. playback_state: 'playing', a stable group_id) match.
2. A scenario where the server emits an initial group/update with group_name set, then a second with group_name: null, and assert the client clears the field.
3. A scenario that exercises a group transition (stopped → playing → stopped) and verifies every state transition arrives as a delta.

Spec references

• "Server → Client: group/update"
• "Contains delta updates with only the changed fields... Fields set to null should be cleared from the client's state."