fix(ci): pin Core integration assembly

[XnLemon]

Summary

• require an exact Stack SHA for Core-triggered integration
• check out and validate that exact Stack revision in backend and browser jobs
• export the prepared secure Nacos proxy image in both assembly paths
• add a structured workflow regression test for revision pinning and image propagation

Fixes the deterministic failure in NeKiro Core Satellite Integration run 31406017187, where Compose rejected the missing NEKIRO_NACOS_SECURE_PROXY_IMAGE before starting either assembly.

Validation

• go test -count=1 ./...
• go test -race ./...
• go build ./...
• go vet ./...
• go mod tidy
• git diff --check