Function getCachedValue

One rule: every symbol write on an event-like object is non-enumerable (via setCachedValue/getOrComputeCachedValue below). Nothing survives an object copy implicitly — spread only copies enumerable own properties. Carry-forward across factory-pipe operations is performed EXPLICITLY by the pipe (pipeFromAsyncArray / EventFactory.chain), which copies every symbol in PRESERVE_EVENT_SYMBOLS (helpers/pipeline.ts) from each step's input to its output after the operation runs. stamp()/sign() additionally re-copy EncryptedContentSymbol via Reflect.has/get/set so a standalone sign(signer)(draft) call (outside any pipe) still preserves plaintext.

This replaces the three-category taxonomy this doc comment used to carry (superseded — see 05.1-symbol-propagation-redesign D-05/D-06): the memo-vs-carry-forward distinction that taxonomy existed to document is now met structurally by the one rule above plus the pipe's explicit carry, not by a hand-maintained category system an author had to keep in sync.

Scope: this helper (getCachedValue/setCachedValue/getOrComputeCachedValue) writes non-enumerable so a spread drops the write unless the pipe carries it forward. The write descriptor's other two flags are load-bearing, not stylistic:

• configurable: true is required because setCachedValue may be called again on the same event/symbol to overwrite a previous memo via Object.defineProperty — redefining a non-configurable property throws a TypeError instead of updating the value.
• writable: true is NOT required by setCachedValue itself: setCachedValue overwrites an existing memo via Object.defineProperty, and configurable: true alone permits redefinition regardless of writable. It is kept so an external event[sym] = x / Reflect.set on a memo still succeeds instead of silently failing.
• Writing via Object.defineProperty (rather than Reflect.set) THROWS a TypeError if the event is frozen, sealed, or otherwise non-extensible, where Reflect.set would have returned false silently. This is deliberate (D-02): a silent write failure here means a stale memo is returned forever, so surfacing the programming error is correct. Consumers that freeze events (e.g. Redux Toolkit / immer freezing state in development) will see a throw where they previously saw silent degradation. getExpirationTimestamp routes through getOrComputeCachedValue, and both EventStore.add and AsyncEventStore.add call it before any replaceable branching — the throw is therefore NOT limited to replaceable events; an ordinary regular-kind event (e.g. a kind-1 note) reaches it on a normal insert. Two early returns do precede the call in both stores, and an event taking either one never reaches it via this path: the kinds.EventDeletion kind check, and the separate this.deletes.check(event) tombstone check (so a kind-1 note that was already deleted returns early too).