Summary

Publish v1.0 of docs/kcenon-system-layout.md, the canonical directory-structure standard for the 8-system kcenon ecosystem (common, thread, logger, container, monitoring, database, network, pacs).

Part of #656. Foundation step — sibling sub-EPICs in #657 (Phases 1-6) reference this document to verify conformance.

Why

• Each of the 8 systems currently uses a different layout (4 patterns for public header location; 2 systems lack include/; CMakeLists.txt sizes range 150-1032 lines)
• A single source-of-truth prevents each migration phase from re-deciding conventions
• Downstream consumers and contributors need one document, not eight

Scope

The document MUST specify:

1. Public header path: include/kcenon/<name>/... with kcenon:: namespace
2. Implementation path: src/
3. Test directory rules: tests/ for unit tests (GTest), integration_tests/ for cross-system
4. CMake module decomposition (extracted from pacs_system, see chore(cmake): extract kcenon-cmake-template from pacs_system #659):
   cmake/options.cmake, compiler.cmake, dependencies.cmake, targets.cmake,
   install.cmake, testing.cmake, examples.cmake, warnings.cmake
5. Forwarding-header policy for breaking moves: [[deprecated]] for one minor release
6. Documented exceptions (e.g., pacs_system retaining Catch2 — see [EPIC] Ecosystem-wide directory structure standardization #657 Non-Goals)

Acceptance Criteria

• docs/kcenon-system-layout.md exists with sections: Layout, Headers, Sources, Tests, CMake, Forwarding, Exceptions
• Standard linked from common_system README (tracked separately in docs(readme): link kcenon-system-layout.md from README #662)
• Master EPIC [EPIC] Ecosystem-wide directory structure standardization #657 body updated to reference v1.0 of the document
• No CMake/source changes in this PR — documentation-only

Out of Scope

• Implementing the cmake template (chore(cmake): extract kcenon-cmake-template from pacs_system #659) or cross-build CI (ci(infra): add ecosystem cross-build workflow #660)
• Migrating any of the 8 systems to the new layout