The TypeScript types in `src/types/opds.ts` declared fresh
`Symbol('content')` / `Symbol('summary')` instances. foliate-js's
`opds.js` declared its own distinct ones, and since Symbols are unique
per call, `metadata[SYMBOL.CONTENT]` always returned undefined — even
though the parser had written the value under a same-named Symbol.
This broke silently in 0.11.1 after foliate-js #14 stopped also setting
a plain `content: string` fallback. For OPDS 1.x feeds (e.g. CWA) the
book description lives in `<entry><summary>`, which foliate-js exposes
only via `[SYMBOL.CONTENT]` — so the description vanished.
Re-export the SYMBOL from foliate-js so consumers read the same Symbol
identities the parser writes.
Huang Xin
708e06a46e