Fail fast on unrunnable Unix host builds

[CoreyRDean]

Non-technical summary

• Stops the Unix helper scripts from pretending Linux can produce a runnable local BlitzForge toolchain today.
• Fails early with a clear explanation instead of leaving contributors with a blitzcc binary that cannot start because its companion libraries were never built.
• Keeps the existing macOS alpha workflow intact while making the unsupported Linux path explicit.

Technical summary

• refactor compile.sh into sourceable helpers plus a main() guard so script-level contract tests can exercise host detection without accidentally launching a build
• add a Linux host guard in compile.sh that rejects unrunnable Unix builds before the CMake step
• add a test.sh preflight check for the required runtime/linker companion libraries so missing host artifacts fail with an explicit message
• add scripts/test_compile_contract.sh to lock the supported-host contract with Darwin success and Linux failure cases
• breaking changes: ./compile.sh now exits early on Linux instead of producing a bin/blitzcc that cannot load runtime/linker at startup

Additional notes

• Trade-off: this chooses honest failure over preserving a misleading partial build artifact.
• Intentionally deferred: shared README/CI wording updates are left for the already-open packaging PRs that touch those same surfaces, to avoid overlapping work in this run.
• Remaining gap: runnable Unix host builds still exist only on macOS until Linux runtime/linker support is implemented.