Rename catalog task APIs to subscribe/unsubscribe

[kixelated]

Summary

The moq_consume_catalog API returns two kinds of handles that are easy to confuse:

1. A task handle from moq_consume_catalog() — represents the ongoing subscription
2. A snapshot handle delivered via the on_catalog callback — represents a specific catalog version

Both are opaque u32 values with "catalog" in the name, making it unclear which handle goes with which close function.

Proposal

Rename the task-side functions to use subscribe/unsubscribe terminology:

Current	Proposed
moq_consume_catalog()	moq_consume_catalog_subscribe()
moq_consume_catalog_close()	moq_consume_catalog_unsubscribe()

The snapshot-side functions keep the simpler naming since that's the pattern C callers expect:

• moq_consume_catalog_free() → moq_consume_catalog_close() (or keep as _free)
• moq_consume_video_config() / moq_consume_audio_config() — unchanged

This is a breaking API change, so it should target the dev branch.