What

Move all .h / .hpp headers from feature directories at the repository root into the canonical location include/kcenon/container/.

Current scattered locations:

• core/ — 21 headers
• internal/ — 16 headers
• integration/ — 1 header
• messaging/ — 1 header

Target:

• include/kcenon/container/core/
• include/kcenon/container/internal/
• include/kcenon/container/integration/
• include/kcenon/container/messaging/

Why

include/ already contains some headers under kcenon/container/, but feature directories still hold 39 headers at the repo root. As a result:

• #include paths are ambiguous: the same type can be reached via multiple roots.
• core/, internal/, integration/, messaging/ cannot be removed at repo root, blocking the EPIC's layout goal.
• Some already-moved headers ended up nested awkwardly (e.g. include/kcenon/container/container/fwd.h); this issue is the right time to also fix the nested container/container/ paths.

Where

• Source: core/, internal/, integration/, messaging/ (header files only — .cpp is out of scope, see #)
• Target: include/kcenon/container/{core,internal,integration,messaging}/
• Affected: every translation unit that #includes the moved headers

How

Technical Approach

1. Move headers preserving subdirectory structure where it exists.
2. Update every #include site in the repo (sources, tests, examples, benchmarks).
3. Update target_include_directories(...) in CMakeLists.txt so the new layout is discoverable from public + private targets.
4. Fix the existing include/kcenon/container/container/... nested paths to flat include/kcenon/container/....
5. Run full build + tests.

Acceptance Criteria

• core/, internal/, integration/, messaging/ contain no header files
• All public/internal headers reachable under include/kcenon/container/
• include/kcenon/container/container/ nested directory removed (flattened)
• Full build green
• All ctest cases pass
• Downstream consumers (pacs, network) still build

Estimated Size

M (~39 headers + many #include sites). Pure mechanical move + path updates.

Part of #531