fix: self-heal missing platform bundle from GitHub Releases (#303) (#335)

Installing from a registry mirror (npmmirror/cnpm) that hadn't mirrored the
per-platform optionalDependency left codegraph failing with "no prebuilt
bundle for <platform>" — npm treats an unfetchable optional dep as success and
silently skips it. The npm-shim now self-heals: when the bundle is missing it
downloads the matching archive from GitHub Releases (checksum-verified, with a
download timeout) and caches it, so a global install works on any registry.

release.yml now publishes SHA256SUMS and triggers an npmmirror sync after
publish. Adds hermetic tests for the shim (resolution, cache reuse, disable
knob, download + checksum match/mismatch/absent).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>