Fail fast on unsupported Unix release packaging

[CoreyRDean]

Non-technical summary

• Stops publish.sh from pretending it can ship a supported Unix release archive on hosts that only build blitzcc.
• Makes the release workflow fail early with a clear explanation instead of packaging an incomplete artifact.
• Keeps the README and CI aligned with the real release surfaces: Windows stable and macOS arm64 alpha.

Technical summary

• Refactors publish.sh host detection into testable helpers and rejects non-macOS release packaging before any staging or zipping work begins.
• Adds scripts/test_publish_contract.sh to lock the host-detection and fail-fast contract, including the unsupported Linux path.
• Runs that contract test in the existing macOS Actions job and documents in ReadMe.md that Unix source builds can stop at ./compile.sh until runtime/linker packaging exists outside macOS.
• Breaking changes: none. Unsupported Linux release packaging now fails explicitly instead of producing a misleading partial archive.

Additional notes

• Trade-off: this chooses honest failure over speculative Linux archive naming until the runtime/linker bundle exists for that platform.
• Deferred follow-up: if Linux becomes a supported release target, publish.sh can grow a real archive path once the runtime/linker artifacts are buildable and distributable there.
• Remaining gap: macOS runtime execution is still alpha/stubbed; this change only hardens the release-packaging boundary around that existing reality.