refactor(container): deprecate legacy include/container/ forwarding headers

[kcenon]

What

Summary

Mark every legacy forwarding header in include/container/ as deprecated and document the removal target in both CHANGELOG.md (downstream-tooling SSOT) and docs/CHANGELOG.md (registry SSOT). Migrate the only in-repo translation unit that still used the legacy path so the in-repo build itself stays warning-free.

Change Type

• Refactor (no functional changes)
• Documentation (CHANGELOG entries)

Affected Components

• include/container/optimizations/fast_parser.h — the only file currently under the legacy include/container/ tree
• tests/fast_parser_integration_tests.cpp — in-repo legacy include migrated to canonical path
• CHANGELOG.md — new "Deprecated" entry under [Unreleased]
• docs/CHANGELOG.md — mirrored "Deprecated" entry under [Unreleased]

Why

Problem Solved

The legacy include/container/ path predates the canonical include/kcenon/container/ layout. Today the same headers can be reached via two roots, and downstream consumers keep depending on the legacy path because nothing has signalled that it is going away. After this PR:

• Downstream code that still uses <container/...> gets a portable build-time warning to migrate.
• CHANGELOG.md records the deprecation and the planned hard-removal release.
• The path is cleared for the actual physical removal at the planned future minor release (out of scope for this PR — deprecation only).

Related Issues

• Closes [#531-3/5] Mark legacy include/container/ forwarding [[deprecated]] #534
• Part of [EPIC] container_system: consolidate scattered headers and standardize layout #531 (EPIC: directory layout normalization)

Where

File	Change
include/container/optimizations/fast_parser.h	Add #pragma message("warning: …") deprecation notice + Doxygen @deprecated block
tests/fast_parser_integration_tests.cpp	Migrate <include/container/...> → <kcenon/container/...>
CHANGELOG.md	Add ### Deprecated section under [Unreleased]
docs/CHANGELOG.md	Mirror the ### Deprecated entry under [Unreleased]

How

Per-header deprecation mechanism

The single forwarding header that currently exists in include/container/ is a pure #include forwarder (no own type aliases / using-decls / function declarations). The C++ [[deprecated]] attribute cannot attach to an #include directive, so attribute-based deprecation is not applicable here. The portable alternative is #pragma message("warning: ..."), which works on GCC, Clang, and MSVC. (#warning is intentionally avoided because not all toolchains in CI accept it consistently — MSVC requires #pragma message.)

Mechanism per header:

Header	Mechanism	Reason
include/container/optimizations/fast_parser.h	#pragma message("warning: …")	Pure #include forwarder; attribute cannot attach

If future sub-issues introduce additional forwarders that re-export type aliases or function declarations, those should prefer [[deprecated("...")]] on the declaration; only fall back to #pragma message when the construct cannot carry an attribute.

Removal target version

Project version is currently 1.0.0 (released 2026-04-16, see CHANGELOG.md ## [1.0.0], vcpkg.json version-semver: "1.0.0", and CMakeLists.txt VERSION 1.0.0). The next minor release is therefore v1.1.0. Per the issue brief, the removal target is documented as "the next minor release after v1.1.0" so the deprecation gets at least one full minor release for downstream consumers to migrate before the legacy path is removed.

This wording is recorded in three places:

• include/container/optimizations/fast_parser.h Doxygen block + #pragma message text
• Root CHANGELOG.md [Unreleased] > Deprecated bullet
• docs/CHANGELOG.md [Unreleased] > Deprecated bullet

In-repo legacy use migration

grep swept the entire repo for #include <container/...> / #include "container/..." patterns that are not the canonical <kcenon/container/...> path. The only hit was tests/fast_parser_integration_tests.cpp:21, which is migrated in this PR to <kcenon/container/optimizations/fast_parser.h>. After this PR the in-repo build will not emit the new deprecation warning, and the warning is reserved for external consumers.

Testing Done

• Local toolchain unavailable in this sandbox (no cmake / g++); relying on CI for build/test verification, same approach used by refactor(container): consolidate headers under include/kcenon/container/ #537 and refactor(container): migrate sources into src/{core,internal,integration,messaging}/ #538.
• Diff is intentionally minimal: only the four files listed above.

Breaking Changes

None. This is a deprecation-only PR — the legacy header still functions and forwards to the canonical location. Downstream consumers will simply see a new build-time warning until they migrate.

Rollback Plan

Revert this single squash commit. The forwarder reverts to its pre-PR pure #include form; the test reverts to the legacy include; the CHANGELOG entries are removed.

Out of Scope (sibling sub-issues of EPIC #531)

• Decomposing CMakeLists.txt into cmake/ modules — issue [#531-4/5] Decompose CMakeLists.txt into cmake/ modules #535
• Doxyfile / install rule rewrite — issue [#531-5/5] Update Doxygen input paths and install rules #536
• Physical removal of the include/container/ directory — happens at the planned future minor release, not in this PR
• Updating grpc/README.md code samples that reference the legacy path — those are docs about deprecated usage and are appropriate to leave until [#531-5/5] Update Doxygen input paths and install rules #536 cleans them up if it touches docs

Checklist

• Every header under include/container/ carries a deprecation marker (#pragma message for the only existing pure #include forwarder)
• CHANGELOG.md lists deprecated header + replacement + removal target version
• docs/CHANGELOG.md mirrors the same entry (per the SSOT mirroring note in CHANGELOG.md)
• No in-repo translation unit uses the legacy paths anymore
• Self-review completed
• Conventional Commits format
• No Claude/AI/Anthropic attribution
• No emojis
• Issue linked with Closes #534 and Part of #531