feat(distribution): Node 26 and cross-platform server packages

[Emran025]

Summary

• Upgrades all GitHub Actions Node.js environments to 26.7.0.
• Adds verified, self-contained server runtime staging for Windows, Linux x64, macOS Intel, and macOS Apple Silicon.
• Adds native Headless packages: DEB, RPM, and tarball for Linux; PKG for macOS; keeps the existing Windows NSIS installer.
• Adds systemd and launchd service lifecycle support with private runtime data and read-only public status receipts.
• Extends release asset staging and signed manifests for Headless artifacts across all supported platforms.

Validation

• cargo test in distribution/crates/accore-server-agent
• node --test scripts/create-release-manifests.test.mjs scripts/stage-desktop-release-assets.test.mjs
• YAML and JavaScript syntax validation
• Linux runtime staging validated against verified upstream digests

Note

macOS packages are intentionally unsigned and unnotarized until Apple Developer ID and notarization credentials are provided.