Main promotion: collect linux/amd64 CUDA receipt from Windows/WSL Nvidia host

[joelteply]

Why

PR #1399 added scripts/main-promotion-gate.sh and aggregate receipt checking. Mac can provide darwin-arm64-metal, but canary to main cannot be promoted until linux/amd64 CUDA and linux/amd64 Vulkan receipts exist for the same SHA.

Windows/WSL Nvidia host command

From a clean WSL checkout:

git fetch origin canary
git switch canary
git reset --hard origin/canary
CONTINUUM_RELEASE_PUSH_IMAGES=1 CONTINUUM_GATE_RUN_HEARTBEAT=1 scripts/main-promotion-gate.sh

Then post the receipt path and output to AIRC.

Validation

• Receipt JSON has gpu_class cuda and role linux-amd64-cuda.
• No CPU fallback accepted.
• Aggregate check still fails until Vulkan receipt also exists, then passes once all required receipts share the same SHA.