v1.11.1-rc6

• CMake/CI — optional ACE discovery (cmake/FindACE.cmake, NO_ACE / prepare_cmake.sh --no-ace); dedicated cell-ace job (with-ace);
• CI — exercise ATLSTL on Windows via with-atl cell (cmake/FindATL.cmake);
• CI — exercise MFCSTL on Windows via with-mfc cell (stlsoft_prepare_mfc_target);
• CI — restore executable bits after artifact download; fail when zero unit tests discovered; do not chmod/run CMake artefacts as examples;
• CMake — prefer tree headers over installed STLSoft (NO_SYSTEM_FROM_IMPORTED); set CMP0177 on CMake ≥ 3.31;
• ACESTL — invoke_ACE_OS_snprintf implemented in terms of stlsoft_C_snprintf(); unit tests for ACE_String_Base / ACE_Time_Value string-access shims;
• TODO.md — restructured 1.11 / 1.12 roadmap; marked ACE/ATL/MFC CI and terse-API items complete;
• Scripts — .sis/project_name.txt; colourised run_all_*.sh / execute_performance_tests.sh; new ctest_cmake.sh; MSYS auto-MinGW in prepare_cmake.sh;
• Applied misc-dev-scripts 0.6.0 editor/Git/.sis drop-in templates on boilerplate;
• Restored historical .gitignore patterns as a sorted union with misc-dev-scripts gold section layout;
• INSTALL.md / README.md — ACE / ACESTL discovery notes;
• test.unit.versions — aligned with 1.11.1-rc6 (_STLSOFT_VER_1_11_1_RC6);
• All unit/component test assertions use xTests Terse API;
• Renamed C-language test/example programs to subject.C form (e.g. test.unit.comstl.string.BSTR_functions.C);
• Fixed stlsoft::unused_return_value_monitor<> copy constructor — clear m_bUsed on the source object (was incorrectly using pointer dereference);
• fix: MinGW compatibility;
• fix: C90 compatibility;
• Fixed stlsoft::string_to_integer_range<>() — call renamed try_string_to_integer_4slre_ helper;
• Added stlsoft::sinteger64 copy constructor;
• stlsoft/api/external/string.h — case-insensitive aliases (STLSOFT_API_EXTERNAL_string_stricmp and kin) for Clang as well as GCC, and for macOS (__APPLE__); added test.unit.stlsoft.api.external.string;