Document community characters + recaps

[KodeStar]

Closes the docs DoD lag from audiosilo-server #38 / audiosilo-frontend #58, which shipped the community characters + story so far recap layer end-to-end (the /meta seam) without the docs catch-up.

Changes

• server/api/reference.md - work.characters / work.recaps added to the GET /libraries/{id}/meta example and documented: the edition-independent position model ({chapter}), the character fields (id/name/aliases/role enum/reveal/description), the recap fields (through/scope/text), and the note that the server passes these straight through from upstream GET /works/{id} but drops the character xref.
• frontend/state-and-data.md - the Characters + Story so far render in BookMetaSection (closed-by-default accordions, no blur; the unit-tested label helpers).
• listening/browsing.md (User Guide) - the user-facing Characters and Story so far sections under 'About this book', framed for spoiler-safety and noting they are community-contributed (so a matched book may still lack them).

Verification

• npm run build green (throws on broken links/anchors/images).
• Field names, the role enum, and the dropped xref verified against the server internal/meta structs, the frontend types.ts + book-meta.tsx helpers, and the audiosilo-meta schema.
• Ran /simplify + /code-review on the diff (per the standing pre-merge process); the review caught an over-claim ('mirrored verbatim') that was corrected to reflect the dropped xref.