fix: package official Node.js runtime artifacts

[rvagg]

Summary

• package official Node.js binaries on amd64, arm64, and s390x without rewriting their host loader or runtime search paths
• retain an Experimental source build for armhf, while supporting the official armv7l artifact on Node.js 22
• select only complete release/nightly artifact sets and embed their published SHA-256 checksums
• provide Node.js 25+'s libatomic.so.1 dependency in an isolated directory and resolve nested Node/npm commands directly inside the Snap
• make the scheduled publisher serialize per branch, remove its self-triggering push event, and keep Git operations out of the generator
• document the artifact, architecture, and publishing model

Refs #54.

Validation

Launchpad recipe nodeedge-runtime-test built commit 5e507d3 successfully on all supported processors:

• amd64: 17m 19s
• arm64: 12m 25s
• armhf: 1h 41m
• s390x: 14m 13s

Artifact checks:

• amd64, arm64, and s390x packaged Node binaries are byte-identical to the checksummed upstream artifacts
• supported binaries retain their host interpreters and contain no RPATH/RUNPATH
• armhf uses the core24 loader and architecture-aware patched RPATH
• amd64 resolves only libatomic.so.1 from the Snap; libc and libstdc++ resolve from the host
• Node.js 27, Temporal, npm, npx, child Node dispatch, npm lifecycle host commands, and a host-built N-API addon passed on amd64
• Yarn is absent
• edge, Node.js 26, 24, and 22 generator modes passed; Node.js 22 selected its official armv7l artifact
• the exact workflow generation and local-repository publishing scripts passed focused harness tests

The intended rollout is edge first, then Node.js 26, followed by Node.js 24.