library: specify sources, scanning and matching

[maxscharwath]

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

Everything upstream of "there is something to play": sources, scan triggers, filename matching, metadata refresh, and what happens when a file moves or disappears.

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/library/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