refactor(container): consolidate headers under include/kcenon/container/

[kcenon]

What

Consolidates all .h headers from feature directories at the repository root into the canonical include/kcenon/container/ tree, and flattens the awkward include/kcenon/container/container/ nested layout that existed in the previous structure.

Affected Components

• internal/*.h (16 files) -> include/kcenon/container/internal/
• internal/async/*.h -> include/kcenon/container/internal/async/
• integration/messaging_integration.h -> include/kcenon/container/integration/
• messaging/message_container.h -> include/kcenon/container/messaging/
• core/*.h forwarders (21 files) deleted - canonical copies were already at include/kcenon/container/
• include/kcenon/container/container/*.h (7 files) flattened to include/kcenon/container/

Why

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

• #include paths were ambiguous: the same type was reachable via multiple roots
• Feature directories at the repo root could not be removed, blocking the EPIC's layout goal
• Some already-moved headers ended up nested awkwardly (e.g. include/kcenon/container/container/fwd.h); this PR also fixes that

Where

• Source: core/, internal/, integration/, messaging/ (header files only - .cpp is out of scope, see [#531-2/5] Source migration to src/{core,internal,integration,messaging}/ #533)
• Target: include/kcenon/container/{,internal,integration,messaging}/
• Affected: every translation unit that #includes the moved headers (sources, tests, examples, benchmarks, integration_tests, fuzz, grpc)

How

Implementation

1. git mv headers preserving subdirectory structure
2. Updated relative #include paths inside canonical headers (e.g., "../value_types.h" -> "value_types.h" after flattening)
3. Updated every #include site in the repo to use <kcenon/container/...> paths
4. Updated CMakeLists.txt source list to reference headers at new locations
5. The legacy repo-root container.h forwarder remains in place (will be deprecated in [#531-3/5] Mark legacy include/container/ forwarding [[deprecated]] #534)

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 (verified by CI)
• All ctest cases pass (verified by CI)
• Downstream consumers (pacs, network) still build (verified by CI)

Out of Scope (handled in sibling sub-issues)

• Moving .cpp files ([#531-2/5] Source migration to src/{core,internal,integration,messaging}/ #533)
• Adding [[deprecated]] to legacy forwarders ([#531-3/5] Mark legacy include/container/ forwarding [[deprecated]] #534)
• Decomposing CMakeLists.txt into cmake/ modules ([#531-4/5] Decompose CMakeLists.txt into cmake/ modules #535)
• Updating Doxyfile or install(DIRECTORY ...) rules ([#531-5/5] Update Doxygen input paths and install rules #536)
• Removing include/container/ forwarding header ([#531-3/5] Mark legacy include/container/ forwarding [[deprecated]] #534)

Verification

Local toolchain unavailable in the working environment; relying on CI for full build and test verification.

Test Plan

1. CI runs: cmake -S . -B build && cmake --build build/ -j
2. CI runs: ctest --test-dir build/ --output-on-failure
3. Verify downstream consumers still build via the kcenon ecosystem CI

Closes #532
Part of #531