Add ForEntityIds for fan-out enrichment (1-event-to-N-entities)

[jeremydmiller]

Summary

Companion API addition for JasperFx/marten#4329. Closes the 1-to-N gap reported in @xander-abn's followup — the existing declarative enrichment chain only models a single referenced entity per event, so a projection that needs to enrich one event with several upstream entities of the same type (e.g. an event carrying a list of foreign-key ids) had to drop down to `EnrichUsingEntityQuery` and write the fan-out by hand.

This adds a first-class declarative variant that mirrors the existing 1-to-1 chain:

```csharp
await group
.EnrichWith()
.ForEvent()
.ForEntityIds(e => e.ProviderIds) // Func<TEvent, IEnumerable>
.AddReferences(); // one References per resolved id
```

What changes

• `EventStep<TEntity, TEvent>.ForEntityIds(...)` — selector returns `IEnumerable`. Returns the new `MultiIdentityStep<TEntity, TEvent, TEntityId>`.
• `ForEntityIdsFromEvent` — companion overload that hands the `IEvent` wrapper to the selector (matches the existing `ForEntityIdFromEvent` shape so callers can also reach headers, sequence, etc.).
• `MultiIdentityStep` — mirrors `IdentityStep`: `AddReferences`, `EnrichAsync`, and `FetchEntitiesAsync`. `AddReferences` emits one `References` per resolved id in order; `EnrichAsync` invokes the callback once per (slice, event, resolved entity) triple. Missing ids are silently skipped (consistent with how `IdentityStep` handles a non-resolving single id).
• `FetchEntitiesAsync` de-duplicates ids before `LoadManyAsync` so the storage layer is never asked for the same id twice within an enrichment.

Test plan

5 new unit tests in `SliceGroupTests`:

• `for_entity_ids_fans_out_one_event_to_many_references` — happy path
• `for_entity_ids_skips_missing_ids` — missing ids don't materialize as null references
• `for_entity_ids_dedupes_loads_for_same_id_across_events` — storage-layer dedupe
• `for_entity_ids_returns_nullo_cache_when_no_ids_extracted` — empty fast path
• `for_entity_ids_enrich_async_invokes_callback_per_resolved_entity` — callback semantics

All 240 `EventTests` pass on net8.0 / net9.0 / net10.0.

Pairs with #206

The cache-eviction correctness fix in JasperFx/jasperfx#206 (just merged) is the prerequisite for using `ForEntityIds` in composite projections at scale — without that fix, a single event referencing more upstream entities than `CacheLimitPerTenant` holds would silently drop the evicted ones onto `LoadManyAsync`, which can't see in-flight upstream writes. Both will ship together in the next `JasperFx.Events` release.

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