contracts: replay workstation Mac-on-Linux polish signals

[mdheller]

Summary

Fresh current-base replay of #81.

This preserves the workstation Mac-on-Linux polish contract surfaces from the stale/non-mergeable Copilot PR while respecting current main, which already had a broader DesktopProfile.macOnLinuxPolish metadata object.

Captured from #81

• WorkstationProfile.validation.polishValidationCommand
• WorkstationProfile.macOnLinuxPolish
• DesktopProfile.appearance
• DesktopProfile.sidebarBookmarks
• workstation and desktop example updates
• docs/workstation-contract-family.md Mac-on-Linux polish boundary documentation

Current-main preservation

The replay preserves current DesktopProfile.macOnLinuxPolish fields already present on main:

• implementationAuthority
• activeFeatures
• validationBackedFeatures
• proposedFeatures
• nonGoals

Boundary

• Additive schema/example/docs contract replay only.
• Implementation authority remains SociOS-Linux/source-os.
• No full macOS parity claim.
• No runtime implementation.
• No package-manager changes.
• No UI asset or proprietary asset dependency.

Supersedes

Supersedes stale/non-mergeable #81 after this replacement lands.