fix vm virtualization detection

[ncode]

Summary

Fixes native VM content validation failures where supported lab guests reported virtual: "physical" and is_virtual: false despite QEMU/KVM indicators being present.

Changes

• Detect QEMU/KVM from DMI/SMBIOS/PCI indicators on OpenBSD, NetBSD, DragonFly BSD, and illumos.
• Detect Windows QEMU/KVM guests from WMI manufacturer/model/BIOS indicators.
• Make the Windows release gate compatible with Windows PowerShell 5.1.
• Add OpenSpec coverage and update the changelog.

Root Cause

The existing virtualization detector only handled Linux, macOS, FreeBSD, OpenBSD's narrow product map, and Windows' older OEM/netkvm patterns. NetBSD, DragonFly, and illumos fell through to physical; OpenBSD and Windows missed the QEMU/SeaBIOS values exposed by the lab VMs.

Validation

• Focused virtualization tests pass.
• Native lab content checks now report virtual: "kvm" and is_virtual: true on OpenBSD, NetBSD, DragonFly BSD, illumos/OmniOS, and Windows.
• Affected release gates pass.
• Full JSON parse passes on affected guests.
• 17 release targets cross-compile.
• go test ./...: 1455 passed.
• go vet ./...: no issues.
• openspec validate fix-vm-virtualization-detection --strict: valid.
• git diff --check: clean.