Add variant matrix enumeration and CI coverage

[NathanNeurotic]

Summary

• add canonical build toggle definitions, output directory handling, and BUILD_CONFIG stamping in the Makefile
• introduce variant enumeration plus multi-target builds and validation for feature toggles, now allowing empty PRINTF overrides to default to NONE for CI compatibility
• embed runtime storage and XFROM IRX objects/imports for runtime MMCE/MX4SIO/XFROM variants, and add a cl workflow that consumes make list-variants to build and upload all variants

Testing

• make list-variants

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[chatgpt-codex-connector]

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