Summary

Phase 0 + Phase 7 of the ecosystem-wide directory structure standardization. This sub-EPIC owns the foundation work — publishing the layout standard, extracting the canonical cmake/*.cmake template from pacs_system, and wiring an ecosystem cross-build CI workflow — and the final verification pass after the other 7 systems migrate.

Part of #657 (master EPIC, see issue body for full phase order).

Why

• common_system sits at the root of the dependency graph; touching it last (Phase 7) avoids cascading rebuilds during the migration
• A single source-of-truth layout document (docs/kcenon-system-layout.md) prevents each system from re-deciding conventions
• The cmake/*.cmake decomposition pattern already proven in pacs_system pays back across 7 other systems once extracted as a template
• An ecosystem cross-build CI workflow is the only practical way to detect when one repo's structural change breaks the other 7

Scope

1. Layout standard document — docs/kcenon-system-layout.md describing the canonical directory structure, header path convention (include/kcenon/<name>/), src/ layout, test directory rules, cmake module decomposition
2. Canonical cmake template — extract pacs_system's 8 cmake/*.cmake modules (options/compiler/dependencies/targets/install/testing/examples/warnings) as a reusable template
3. Ecosystem cross-build CI workflow — .github/workflows/ecosystem-cross-build.yml that fetches all 8 systems and verifies a downstream change does not break siblings
4. Phase 7 verification — after the other 7 sub-EPICs close, audit common_system itself against the standard and apply any final adjustments

Child Issues

• docs(layout): publish kcenon-system-layout.md v1.0 #658 — Publish docs/kcenon-system-layout.md v1.0
• chore(cmake): extract kcenon-cmake-template from pacs_system #659 — Extract kcenon-cmake-template/ from pacs_system cmake modules
• ci(infra): add ecosystem cross-build workflow #660 — Add ecosystem cross-build CI workflow
• chore(audit): Phase 7 standard-conformance audit of common_system #661 — Final standard-conformance audit of common_system itself (BLOCKED until Phases 1-6 in [EPIC] Ecosystem-wide directory structure standardization #657 close)
• docs(readme): link kcenon-system-layout.md from README #662 — Update README to link to the standard document

Acceptance Criteria

• Standard document merged and linked from each of the 8 system READMEs
• cmake template repository (or directory) usable as drop-in by sibling systems
• Cross-build CI passes on PRs that touch any of the 8 systems
• common_system itself conforms to the standard with no exceptions
• Master EPIC body updated with final verification note