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 IRX objects and imports for runtime MMCE/MX4SIO variants, and add a cl workflow that consumes make list-variants to build and upload all variants

Testing

• make list-variants

---

Codex Task

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, you can upgrade your account or add credits to your account and enable them for code reviews in your settings.