Reduce allocations on event/request hot paths

[Copilot]

Profiling-driven cleanup of the publish/dispatch hot paths in RedisEventBusImpl and RequestResponseBusImpl. Wire format and public API are unchanged.

Streaming envelope serialization

Publish previously did encodeToJsonElement (full JsonElement tree allocation) followed by encodeToString (re-walks the tree). Replaced with encode-only KSerializers that stream the typed payload straight into the output:

@OptIn(ExperimentalSerializationApi::class)
private class EventEnvelopeSerializer(
    private val dataSerializer: KSerializer<RedisEvent>
) : KSerializer<EventEnvelopePayload> {
    override val descriptor = buildClassSerialDescriptor("EventEnvelope") {
        element<String>("eventClass")
        element("eventData", dataSerializer.descriptor)
    }
    override fun serialize(encoder: Encoder, value: EventEnvelopePayload) {
        encoder.encodeStructure(descriptor) {
            encodeStringElement(descriptor, 0, value.eventClass)
            encodeSerializableElement(descriptor, 1, dataSerializer, value.event)
        }
    }
    override fun deserialize(decoder: Decoder) = error("encode-only")
}

Element names match the existing data-class envelopes so JsonNamingStrategy.SnakeCase produces identical bytes. The receive path still uses the data-class envelopes (concrete payload type unknown until eventClass is read).

Other changes

• Single dispatch coroutine per message. handleIncomingMessage now launches one coroutine that iterates handlers, instead of one per handler. Per-handler exception isolation preserved via try/catch.
• Skip redundant responseTypeRegistry writes. containsKey short-circuits the per-request putIfAbsent once the response type has been seen.
• Coalesced registrationLock.write blocks in both buses — one write per handler, no inconsistent intermediate state.
• Negative-result caching in KotlinSerializerCache. Added a MISSING sentinel inside the ClassValue so classes without a serializer aren't re-resolved on every lookup.
• Removed the now-unused serializeEvent / serializeRequest / serializeResponse helpers and the EventEnvelope.forEvent / RequestEnvelope.forRequest / ResponseEnvelope.forResponse factories.

Not done

• (2) wire-format change to a delimited form, (4) processStreamEvent split avoidance, (9) Flux.interval polling, (10) counted-loop iteration — out of scope for the recommended subset.
• (8) LuaScriptExecutor cached-SHA fast-path — Aedile's getIfPresent is suspend and .underlying() is parameterized over Deferred<V>, not V. Left as-is pending a way to verify against a working build.

Build note

The repo's settings plugin (dev.slne.surf.api.gradle.settings, hosted on repo.slne.dev / reposilite.slne.dev) isn't reachable from this sandbox, so changes were not compiled here.