Summary

Phase 3 of the ecosystem-wide directory structure standardization. container_system distributes public headers across 5+ locations (include/kcenon/container/, include/container/, core/, internal/, integration/, messaging/). Consolidate to a single include/kcenon/container/, move implementations to src/, and decompose the 750-line CMakeLists.txt.

Part of kcenon/common_system#657 (master EPIC).

Why

• 5+ header locations make #include choice ambiguous — the same type can be reached via multiple paths
• Source files mixed with headers in feature directories block clean install rules
• Legacy include/container/ forwarding adds duplication that can never be removed without a deliberate migration

Scope

1. Move all .h from core/, internal/, integration/, messaging/ into include/kcenon/container/{core,internal,integration,messaging}/
2. Move all .cpp into src/{core,internal,integration,messaging}/
3. Mark include/container/ forwarding headers [[deprecated]]; schedule removal for next minor release
4. Decompose CMakeLists.txt into cmake modules per canonical template
5. Update Doxygen input paths to point at the new locations

Child Issues

• [#531-1/5] Header consolidation to include/kcenon/container/ #532 — Header consolidation to include/kcenon/container/
• [#531-2/5] Source migration to src/{core,internal,integration,messaging}/ #533 — Source migration to src/
• [#531-3/5] Mark legacy include/container/ forwarding [[deprecated]] #534 — Legacy forwarding deprecation
• [#531-4/5] Decompose CMakeLists.txt into cmake/ modules #535 — CMakeLists.txt decomposition
• [#531-5/5] Update Doxygen input paths and install rules #536 — Doxygen + install rule updates

Execution order: #532 → #533 → #534 → #535 → #536 (sequential, dependency-ordered).

Acceptance Criteria

• Single canonical location for all public headers
• core/, internal/, integration/, messaging/ no longer at repo root
• Forwarding headers [[deprecated]] with removal note in CHANGELOG
• cmake modules match the canonical template
• All tests plus downstream consumers (pacs, network) pass