feat(entities): auto-refetch when realtime broadcast is truncated

[theunreal]

Why

Realtime entity broadcasts have a transport size limit. Records with very large fields (e.g. base64-encoded media, long markdown) can exceed the limit and get dropped — subscribers see stale state until refresh.

The Base44 backend now slims oversize broadcasts to fit under the limit and signals it with _oversize: true on the event data. Without this SDK change, customer apps that read the truncated fields would render placeholder text until refresh.

This change makes the recovery transparent: the SDK detects _oversize and refetches the full record over HTTP before invoking the user callback. Existing app code keeps working with no changes.

What

In entities.X.subscribe(callback):

1. Parse the realtime event as before.
2. If event.type !== "delete" and event.data._oversize, call the entity GET endpoint and replace event.data with the response.
3. Concurrent subscribers in the same browser fan out to a single HTTP call — debounced by (entityName, id, timestamp) via an in-flight Map<string, Promise<T>>. The cache entry clears 5 s after the promise settles so late subscribers can still piggy-back.
4. On HTTP failure: log a warning and fall through with the stub payload. UX is no worse than today's behavior on dropped messages.
5. Delete events skip the refetch — the record no longer exists.

Wire contract is preserved: the server always emits data as an object containing at minimum id, so older SDK clients that don't know about _oversize continue to receive a structurally valid event.

Tests

tests/unit/entities-subscribe.test.ts — 5 new tests, all 11 pass:

• refetches full record over HTTP when data._oversize is true
• does NOT refetch on delete events even if _oversize is set
• does NOT refetch when _oversize is absent
• falls through with stub payload + warning when refetch fails
• debounces concurrent refetches for the same (entity, id, timestamp) — only one HTTP call

Full suite: 158 / 158 pass. `npm run lint` and `npm run build` clean.

Backward compatibility

• Existing subscribers: no behavior change for events without _oversize. Pure passthrough.
• Older SDK versions: receive the stub payload as-is — render placeholder strings until refresh, same as today's behavior. Customer apps pick up the new behavior on their next rebuild.
• Server without slimming: _oversize flag never appears, so this code path is dormant.

Versioning

Recommend a minor bump (0.8.x → 0.9.0) since this is additive behavior.

[theunreal]

Superseded by #167 — recreated from a base-repo branch so that secrets/OIDC-dependent checks (claude-review, claude-docs-draft, publish-preview) can run.