Stand up GCP emulators as a cheap target for Google Cloud CD primitives

Part of the **Continuous Delivery Vision** ([#8](https://github.com/ChrisonSimtian/Fallout/milestone/8)); tracked under **CD: Google Cloud (emulators)** (#16). Sibling of the Azure/Topaz (#281) and AWS/LocalStack tracks.

## Idea

Build and exercise **GCP-specific CD primitives** against Google's local service **emulators** — no real GCP spend.

## Caveat

GCP has no single all-in-one emulator like LocalStack/Topaz. Coverage is per-service: `gcloud` emulators (Pub/Sub, Firestore, Datastore, Bigtable, Spanner) + `fake-gcs-server` for storage. Part of this issue is assessing whether the emulator coverage is good enough for the CD targets we want, or whether some paths need a real (free-tier) project.

## Rough scope (TBD)

- [ ] Map the GCP CD targets we care about → available emulators; flag gaps.
- [ ] Document the local emulator setup for dev/test.
- [ ] Prototype GCP CD building blocks against emulated services (start narrow).
- [ ] Decide graduate-to-primitive vs. sample.

Long-horizon, no release commitment. Demand-driven.