fix: handle GPU-less NVIDIA release runners

[wimi321]

Summary

• distinguish a missing KataGo binary from an installed NVIDIA engine that cannot start without CUDA
• keep NVIDIA packaged-app smoke strict for assets, checksums, release readiness, and all unrelated diagnostics
• accept only the expected GPU-less GitHub runner limitation after NVIDIA package integrity passes
• add regression coverage proving standard builds and real asset failures remain blocking

Verification

• pnpm check
• pnpm check:teacher-quality
• pnpm check:release-notes-i18n
• pnpm check:website

Release context

This fixes the remaining v0.4.20 NVIDIA packaged-app smoke blocker. The GitHub Windows runner has no NVIDIA/CUDA runtime, while the packaged CUDA engine, model, version, and checksums are independently validated before this smoke step.