* refactor(integration): switch to KCENON_WITH_* macros in source files

- Include feature_flags.h header in all integration entry points
- Replace BUILD_WITH_THREAD_SYSTEM with KCENON_WITH_THREAD_SYSTEM
- Replace BUILD_WITH_CONTAINER_SYSTEM with KCENON_WITH_CONTAINER_SYSTEM
- Replace BUILD_WITH_MONITORING_SYSTEM with KCENON_WITH_MONITORING_SYSTEM
- Replace BUILD_WITH_COMMON_SYSTEM with KCENON_WITH_COMMON_SYSTEM
- Update comments to reference new macro names

Part of #335: Align integration gating with unified flag contract

* refactor(headers): switch to KCENON_WITH_* macros in header files

- Include feature_flags.h in all public headers
- Replace BUILD_WITH_THREAD_SYSTEM with KCENON_WITH_THREAD_SYSTEM
- Replace BUILD_WITH_CONTAINER_SYSTEM with KCENON_WITH_CONTAINER_SYSTEM
- Replace BUILD_WITH_MONITORING_SYSTEM with KCENON_WITH_MONITORING_SYSTEM
- Replace BUILD_WITH_COMMON_SYSTEM with KCENON_WITH_COMMON_SYSTEM
- Update documentation comments to reference new macro names

Part of #335: Align integration gating with unified flag contract

* build(cmake): switch compile definitions to WITH_*_SYSTEM

Update all target_compile_definitions() to use WITH_*_SYSTEM
instead of BUILD_WITH_*. This allows feature_system_deps.h to
properly detect and define KCENON_WITH_* macros.

Changes:
- BUILD_WITH_CONTAINER_SYSTEM -> WITH_CONTAINER_SYSTEM
- BUILD_WITH_THREAD_SYSTEM -> WITH_THREAD_SYSTEM
- BUILD_WITH_LOGGER_SYSTEM -> WITH_LOGGER_SYSTEM
- BUILD_WITH_COMMON_SYSTEM -> WITH_COMMON_SYSTEM
- BUILD_WITH_MONITORING_SYSTEM -> WITH_MONITORING_SYSTEM

BUILD_WITH_* option() declarations are preserved for backward
compatibility with existing build scripts.

Part of #335: Align integration gating with unified flag contract

* test: switch to KCENON_WITH_* macros in test files

Update test files and verify_build.cpp to use feature_flags.h header
and KCENON_WITH_* macros instead of BUILD_WITH_* macros for
consistent feature detection across the codebase.

- Add kcenon/common/config/feature_flags.h include
- Replace BUILD_WITH_THREAD_SYSTEM with KCENON_WITH_THREAD_SYSTEM
- Replace BUILD_WITH_CONTAINER_SYSTEM with KCENON_WITH_CONTAINER_SYSTEM
- Replace BUILD_WITH_LOGGER_SYSTEM with KCENON_WITH_LOGGER_SYSTEM
- Replace BUILD_MESSAGING_BRIDGE with KCENON_WITH_MESSAGING_BRIDGE

Part of #335

* docs: update code examples to use KCENON_WITH_* macros

Update documentation code examples to use KCENON_WITH_* macros
instead of BUILD_WITH_* for feature detection in source code.

CMake command-line options (-DBUILD_WITH_*) remain unchanged as
they are user-facing configuration options.

Updated files:
- README.md, README_KO.md: Integration example
- docs/INTEGRATION.md, docs/INTEGRATION_KO.md: Feature check examples
- docs/FEATURES.md: Dual API support example
- docs/API_REFERENCE_KO.md: Thread system adapter examples
- docs/integration/with-common-system.md: Concepts integration
- docs/integration/with-monitoring.md: Monitoring adapter example
- docs/implementation/02-dependency-and-testing.md: Test examples

Part of #335

* fix: add local feature_flags.h for standalone builds

Create network_system's own feature_flags.h that works both with
and without common_system dependency:

- When common_system is available (WITH_COMMON_SYSTEM defined),
  include common_system's feature_flags.h
- When common_system is not available, define KCENON_WITH_* macros
  locally based on CMake definitions (WITH_*_SYSTEM)

Update all source and header files to use the local feature_flags.h
at <kcenon/network/config/feature_flags.h> instead of requiring
common_system's header.

This fixes standalone builds (Minimal Build) that were failing due
to missing common_system headers.

Part of #335

* fix(cmake): add KCENON_HAS_COMMON_EXECUTOR for thread_system integration

When thread_system is built with common_system, it uses
KCENON_HAS_COMMON_EXECUTOR=1 which affects the class layout of
thread_pool (conditional inheritance from IExecutor interface).

network_system must define the same macro when including thread_system
headers to ensure consistent class layout across compilation units.
Without this, Windows MSVC fails with LNK2019 unresolved external
symbol errors for thread_pool::is_running().

* docs: update code examples to use KCENON_WITH_* macros

Update remaining code examples in documentation to use the new
KCENON_WITH_* macro style instead of the old BUILD_WITH_* or
#ifdef patterns, consistent with the codebase changes.

* fix(cmake): use WITH_*_SYSTEM macros in integration tests

Fix ODR violation that caused ServerStartupOnUsedPort test to abort.
The integration tests were defining BUILD_WITH_* macros but
feature_flags.h only detects WITH_*_SYSTEM macros. This caused
KCENON_WITH_COMMON_SYSTEM to be 0 in tests but 1 in the library,
resulting in class layout mismatch (messaging_server has conditional
members based on KCENON_WITH_COMMON_SYSTEM).

Changes:
- BUILD_WITH_COMMON_SYSTEM -> WITH_COMMON_SYSTEM
- BUILD_WITH_LOGGER_SYSTEM -> WITH_LOGGER_SYSTEM
- BUILD_WITH_THREAD_SYSTEM -> WITH_THREAD_SYSTEM
- Add missing WITH_CONTAINER_SYSTEM

This aligns integration tests with all other test CMakeLists.txt
which already use WITH_*_SYSTEM macros.

* docs: add CHANGELOG entries for KCENON_WITH_* feature flag unification

Document the feature flag changes made in PR #336:
- Switch from BUILD_WITH_* to KCENON_WITH_* macros
- Add feature_flags.h for unified feature detection
- Fix ODR violation in integration tests

Closes #335

* fix(cmake): use *_SYSTEM_INCLUDE_DIR for integration test definitions

Fix ODR violation that caused "corrupted size vs. prev_size" error
on Ubuntu Debug and test failures on macOS Release.

The integration_tests were using BUILD_WITH_* CMake options to
determine compile definitions, but these don't always match the
actual system availability detected by NetworkSystemIntegration.cmake.

Changes:
- Use *_SYSTEM_INCLUDE_DIR variables instead of BUILD_WITH_* options
  to detect actual system availability
- Add KCENON_HAS_COMMON_EXECUTOR=1 when both thread_system and
  common_system are available (required for consistent thread_pool
  class layout)

This aligns integration_tests compile definitions with the pattern
used in tests/CMakeLists.txt and tests/integration/CMakeLists.txt.

Part of #335

* fix(test): use 127.0.0.1 instead of localhost to avoid IPv6 delays

On macOS, localhost resolves to IPv6 (::1) first, then falls back
to IPv4 (127.0.0.1) when the connection fails. This fallback process
can take 5-10 seconds, causing test timeouts in CI environments.

Changes:
- Replace "localhost" with "127.0.0.1" in ConnectClient()
- Replace "localhost" with "127.0.0.1" in ConnectAllClients()
- Increase CI timeout from 2-3 seconds to 5 seconds for consistency

This fixes ServerShutdownWithActiveConnections test failures on
macOS Release builds.

Part of #335

* docs: update CHANGELOG with ODR and IPv6 fix details

Add details about the ODR violation fix and macOS IPv6 delay fix
to the KCENON_WITH_* feature flag unification section:
- *_SYSTEM_INCLUDE_DIR pattern for consistent macro definitions
- KCENON_HAS_COMMON_EXECUTOR for thread_pool class layout
- 127.0.0.1 instead of localhost to avoid IPv6 fallback delays

Part of #335

* fix(server): prevent heap corruption when start_server fails

When start_server() fails due to port binding errors (e.g., address
already in use), previously created resources (io_context_, work_guard_)
were not cleaned up. This caused heap corruption during object destruction.

Changes:
- Add resource cleanup in start_server() catch blocks to release
  partially created resources before returning error
- Add explicit resource cleanup in destructor to handle cases where
  stop_server() returns early due to is_running_ being false

This fixes "corrupted size vs. prev_size" errors in
ConnectionLifecycleTest.ServerStartupOnUsedPort on Linux Debug builds.

* docs: add changelog entries for messaging_server resource cleanup fix

Document the heap corruption fix in both English and Korean changelogs.