fix: clean build error output

[Sunrisepeak]

Summary

• Keep default build failures focused on user diagnostics by filtering Ninja progress, failed command wrappers, private runtime env prefixes, and mcpp-injected Clang unused-argument warnings.
• Move toolchain runtime library paths out of generated build.ninja and scope them onto the Ninja subprocess instead.
• Preserve raw Ninja details in --verbose, and keep fast-path compile/link failures from falling back into a second full prepare.
• Add an E2E regression covering default and verbose build-error output.

Closes #78

Test plan

• mcpp build --no-cache
• MCPP=$PWD/target/x86_64-linux-gnu/4d24c8b57fdbbbb4/bin/mcpp PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 bash tests/e2e/01_help_and_version.sh
• MCPP=$PWD/target/x86_64-linux-gnu/4d24c8b57fdbbbb4/bin/mcpp PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 bash tests/e2e/05_errors.sh
• MCPP=$PWD/target/x86_64-linux-gnu/4d24c8b57fdbbbb4/bin/mcpp PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 bash tests/e2e/48_build_error_output.sh
• MCPP_HOME=/home/speak/.mcpp /home/speak/workspace/github/mcpp-community/mcpp/target/x86_64-linux-gnu/4d24c8b57fdbbbb4/bin/mcpp build --no-cache in /home/speak/test/mcpp/helloworld, verified source diagnostic appears once and no LD_LIBRARY_PATH=, FAILED:, Ninja progress, or unused-argument warning noise
• temp successful project: verified generated build.ninja contains no toolenv/LD_LIBRARY_PATH, .build_cache records runtime env metadata, and the third build uses the fast path
• git diff --check