Add support for arrangement_clip (new namespace)

[PhotonicVelocity]

Description

Adds a new /live/arrangement_clip/* namespace that mirrors the existing /live/clip/* API but targets clips in Arrangement view (via track.arrangement_clips[clip_index]). This enables reading and writing arrangement clip properties and MIDI notes without overloading the session‑clip endpoints.

Resolves #124

Checklist

• The title is descriptive and summarises the new changes
• The code and any comments are consistent with the current code style
• For any new or modified API endpoints, I have added appropriate documentation to README.md
• For any new or modified API endpoints, I have added unit tests covering the new functionality
• I have verified that all unit tests pass (see CONTRIBUTING.md for guidance)

No tests yet because creating arrangement clips is handled by some other open PRs. The current test suite can just add clips to session view. But I did check that this works.