Fix streaming generation compatibility

[wonderstone]

Summary

• add a top-level num_hidden_layers field to VibeVoiceStreamingConfig for newer transformers generation helpers
• keep the fix isolated to the streaming configuration surface

Why

Recent transformers generation helpers expect num_hidden_layers to be available from the top-level config. Without it, the streaming generation path can break on newer dependency versions.

Notes

• the fork branch was rebased onto the latest wonderstone/main
• the cache initialization compatibility logic already exists upstream on the fork base, so this PR only carries the remaining config-level fix