Fix Objective-C source routing in Ninja backend

[Sunrisepeak]

Summary

• route .m Objective-C sources through the C/Objective-C ninja rule instead of cxx_object
• add a focused ninja backend regression test that verifies .m uses c_object and package-owned cflags
• bump mcpp to 0.0.41 and document the fix

Validation

• mcpp test -- --gtest_filter=NinjaBackend.ObjectiveCSourceUsesCObjectRuleAndCFlags (red before the fix, green after)
• MCPP=$(realpath "$(find target -type f -name mcpp -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)") timeout 600 bash tests/e2e/26_c_language_support.sh
• mcpp test
• git diff --check