Protect blitzcc target-flag workflow coverage

[CoreyRDean]

Non-technical summary

This hardens the developer and CI workflows around blitzcc target selection. The repository now depends on the -target flag for documented local testing and recent macOS alpha work, so this change adds explicit checks that accepted targets still compile and unsupported targets still fail clearly.

Why this matters now: the -target parser was added only recently, after the documented workflow had already drifted into a broken state. Locking that contract into the test entrypoints makes the next regression show up immediately in the same scripts contributors and CI already run.

After this change, the system is better because target-selection regressions fail fast instead of silently breaking test.sh, test.bat, or the host-specific CI assumptions they encode.

Technical summary

• Added explicit -target contract checks to test.sh before the existing suite runs.
• Verified the host alias still compiles on Unix hosts, the native macOS target still compiles on Darwin, and an unsupported foreign target is rejected with the expected error.
• Added matching Windows-side checks to test.bat for host, windows-x86, and rejected macos-arm64 usage.
• Reused the existing test entrypoints instead of introducing a parallel harness, so the new coverage runs anywhere the repository already runs its test scripts.
• Preserved the existing macOS alpha runtime smoke behavior while consolidating temporary artifacts under one cleanup directory.
• Breaking changes: none.

Additional notes

• Trade-off: this improves workflow-contract coverage without introducing a dedicated CLI unit-test harness; that keeps the scope small and aligned with current repo norms.
• Deferred follow-up: if the project later adds more real codegen targets, these script-level assertions should grow with that matrix or move into a more formal CLI test harness.
• Remaining gap: Windows-side execution of the new batch assertions was reviewed statically in this macOS environment, but not run live here because cmd.exe is unavailable on this host.