Separate Music Studio runtime boundary

[0YHR0]

What changed

• moved the Music Studio runtime service and deterministic provider adapters into agentmesh.packs.music_studio
• moved its HTTP routes and focused workspace assets into the scenario package while preserving all public URLs
• kept compatibility re-exports for pre-alpha Python import paths
• documented phase 2 of the Pack boundary and added boundary identity tests

Why

Music Studio needs to become an independently versioned scenario without leaking business-specific behavior into the AgentMesh operating core. This establishes the in-package physical boundary before introducing a trusted external Pack loading protocol.

Validation

• ruff check src tests
• 419 non-PostgreSQL tests passed
• 83.11% line coverage (80% gate)
• wheel build verified to contain all Music Studio HTML/CSS/JS assets