Summary

Add .github/workflows/ecosystem-cross-build.yml to common_system that fetches all 8 sibling repositories and verifies a structural change in any one does not break the others.

Part of #656. Depends on the layout standard from #658.

Why

• Touching layout/cmake in one repo can silently break sibling consumers (e.g., #include <kcenon/...> paths)
• Without an automated cross-build, regressions are caught late by manual integration
• A CI gate enforces the standard rather than relying on review discipline

Scope

1. Workflow triggers on push/PR to common_system, and (optionally) on repository_dispatch from sibling repos
2. Matrix fetches each of the 8 systems at a known ref (default branch unless overridden)
3. Builds the dependency graph in leaf-to-root order: pacs -> database -> container -> network -> monitoring -> thread -> logger -> common
4. Reports per-system build status with logs
5. Documented escape hatch for releases (label skip-cross-build)

Acceptance Criteria

• Workflow file exists and is referenced from docs/kcenon-system-layout.md (docs(layout): publish kcenon-system-layout.md v1.0 #658)
• Successful run on a no-op PR within 20 minutes total
• Failure reproducible and human-debuggable from job logs
• README has a one-line note about the gate
• Documented in the layout standard

Out of Scope

• Cross-repo change-coordination tooling beyond CI
• Automatic ref-pinning across repos