PR Summary

Resolves the in-repo half of #24076 (DSR for Debian 12 arm64). We already build a linux-arm64 self-contained binary and a tar-arm64 tarball, but the only .deb we ship is amd64. This adds a deb-arm64 packaging path that consumes the existing arm64 signed binary drop and emits a deb whose control field reports Architecture: arm64 and whose filename ends in _arm64.deb.

PR Context

The packaging logic itself was almost free — New-DebPackage already takes HostArchitecture and writes it verbatim into the deb control file and output filename, and Debian uses arm64 natively (no aarch64 translation like RPM needs). So the new switch arm in Start-PSPackage is a clone of 'deb' with HostArchitecture = "arm64".

Pipeline-side:

• linux-package-build.yml learns the deb-arm64 packageType and maps it to the Signed-linux-arm64 drop folder
• PowerShell-Packages-Stages.yml adds a deb_arm64 job that consumes the existing drop_linux_build_linux_arm64 / drop_linux_sign_linux_arm64 artifacts (the same ones feeding tar-arm64 today)
• release-validate-packagenames.yml widens the deb regex from _amd64\.deb to _(amd64|arm64)\.deb

I deliberately did not set hostArchitecture: arm64 on the new job's pool. Per OneBranch's ARM64 Build Hosts guidance, that flag is for native compilation of arm64 code; we're just running dpkg-deb over already-built arm64 binaries, which is what tar-arm64 does today on the default amd64 linux pool. The existing signing step's files_to_sign: '**/*.rpm;**/*.deb' glob already picks up the new artifact with no change.

The remaining DSR checklist items (Docker image in PowerShell/PowerShell-Docker, PMC publish, lifecycle and install-doc updates in MicrosoftDocs/PowerShell-Docs) live outside this repo and are tracked on #24076.

Drafted by Copilot (Claude Opus 4.7 (High reasoning)).

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge. If this PR is a work in progress, please open this as a Draft Pull Request and mark it as Ready to Review when it is ready to merge.
• Breaking changes
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  • Experimental feature(s) needed
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• User-facing changes
  • Not Applicable
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  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed