docs: plan multi-project management

[britle20]

Summary

• Adds the multi-project management feature spec at docs/features/22-multi-project-management.md.
• Documents the current single-project limitation and the proposed browser-local project collection model.
• Updates product, architecture, data model, UI, testing, README, and PLANS references.
• Moves Active Issue Order from stale Arrangement WAV export #50 to Multiple local project management #59.

Changes

• Defines transport bar project menu scope: create, switch, rename, delete.
• Defines persistence direction: stable project IDs, separate active project ID, scoped imported sample blobs, single-project migration where practical.
• Keeps cloud sync, file export/import, duplication, and dashboard UI out of scope.

Testing

• git diff --check

Screenshots

Not applicable. Documentation-only planning PR.

Notes / Limitations

• This PR does not implement production code.
• Project duplication remains deferred unless it is trivial after the base workflow exists.

Refs #59