playback: specify direct play, fallbacks and resume

[maxscharwath]

Epic for the playback domain. Spec: docs/spec/playback/README.md

The core product promise. The conditions for direct play, the ordered fallbacks when it is impossible, and the resume semantics across devices.

How this epic works

Implementation and specification work hangs off this issue as sub-issues, so progress rolls up on the board without a hand-maintained checklist. Do not copy spec text into sub-issues — link to the section.

Done when

• Every section of docs/spec/playback/README.md carries a status other than DRAFT
• The must-answer list in that file is empty
• Behaviour that ships in this domain matches what the spec claims