fix: honor discovery-mode CLI options

Summary

• make the standalone CLI default to test discovery in the current directory
• forward --filter and --no-color to normal and coverage-backed zig test child processes
• reject discovery options that cannot currently be honored instead of silently ignoring them
• validate coverage-tool and job-count values
• add an end-to-end fixture proving a filtered-out failing test is not executed
• document the current discovery-mode capability boundary

Verification

• zig fmt --check .
• zig build
• zig build test
• zig build examples
• zig build -Dtarget=x86_64-linux-gnu
• direct filtered discovery run against the integration fixture
• direct validation of unsupported reporter and timeout exit status

Dependency

This PR is stacked on #16 so its diff contains only issue #2. After #16 lands, it can be retargeted to main.

Closes #2