Prototype a C or assembly fixed-point kernel compatibility path

[tsotchke]

Goal
Create a compatibility path for lowering the BASIC fixed-point kernels to C or assembly while preserving GPT2-BASIC file formats and test vectors.

Acceptance criteria

• Prototype covers a narrow kernel first, such as fixed-point dot product, token scoring, or output-head scan.
• Host vectors compare BASIC/Python reference output against the new kernel path.
• Documentation explains this as a portability path, not a replacement for the BASIC proof runtime.
• The preview release remains DOS/BASIC unless a separate port earns evidence.

Evidence gate
python3 -m unittest tests.test_quality_gate
python3 scripts/export_gpt2_basic_vectors.py
python3 scripts/audit_exported_models.py --self-test
python3 scripts/verify_preview_artifacts.py --self-test

Boundary
No physical hardware validation is claimed by this issue.