chore(tests): move integration_tests under tests/integration_tests

[kcenon]

Closes #686

Part of #683.

Summary

Move integration_tests/ to tests/integration_tests/ so the test layout matches the ecosystem-wide tests/{unit,stress,integration_tests,benchmarks}/ standard. Update tests/CMakeLists.txt to add the subdirectory under the existing THREAD_BUILD_INTEGRATION_TESTS gate, and remove the legacy top-level branch from add_tests_subdirectory() in cmake/thread_system_targets.cmake.

Changes

• git mv integration_tests tests/integration_tests (25 files relocated)
• tests/CMakeLists.txt: add gated add_subdirectory(integration_tests) block; document layout in header comment.
• cmake/thread_system_targets.cmake: remove the now-dead top-level integration_tests branch from add_tests_subdirectory().
• scripts/run_valgrind.sh: update the binary path from build/integration_tests/ to build/tests/integration_tests/ since CMake places executables in their source-relative directory.
• .github/workflows/integration-tests.yml: update two hashFiles() cache keys to point at the new CMakeLists.txt path.
• docs/TRACEABILITY.md, docs/guides/COVERAGE_GUIDE.md: refresh active path references.

Test Plan

• cmake -DTHREAD_BUILD_INTEGRATION_TESTS=ON --preset debug should configure cleanly
• cmake --build --preset debug should build integration_tests / smoke_tests / performance_tests
• ctest --test-dir build-debug -R integration should pass

Local toolchain (cmake, g++) is unavailable in this sandbox, so build verification was done by static inspection only (control-flow tokens balance, referenced include directories exist, no orphan paths). Full build verification will run on the eventual main-targeted release PR per ci-resilience policy.

Out of Scope

• Header migration to include/kcenon/thread/ -- handled in refactor(layout): migrate legacy headers into include/kcenon/thread/ with deprecated forwarding #684 (merged)
• CMake auto-detection cleanup -- handled in refactor(build): remove USE_LEGACY_BUILD auto-detection branching #685
• Downstream consumer verification -- handled in ci(verify): downstream consumer builds (logger, monitoring, network, database) #687

Verification of Source File Includes

Source files under tests/integration_tests/{smoke,integration,scenarios,performance}/ use #include "../framework/...". These are relative to the source file's own directory and remain correct after the move because framework/ is still a sibling. CMake CMAKE_SOURCE_DIR references in the moved CMakeLists.txt continue to resolve to the repo root and were left unchanged.