Preserves the full GPU source + spike artifacts at the point the
extraction plan was locked. After @qkstat/rag v0.1.0 ships from
mojo-addon-examples, the spike/gpu-fatbin branch can be deleted; this
tag keeps the code referenceable by commit reach.

Contents at this tag:
- src/gpu/lib.mojo, src/addon/gpu_linalg.mojo (GPU addon source)
- tests/gpu-matmul.test.js, examples/gpu-matmul.js
- build.sh + index.js GPU half
- spike/gpu-fatbin/FINDINGS.md (Path B validation on H100)
- spike/gpu-fatbin/EXTRACTION-PLAN.md (hand-off for @qkstat/rag)
- spike/gpu-fatbin/runpod.sh (validation harness)