workstation-v0 world-class SourceOS parity plan

[mdheller]

Closes #155.

What changed:

• Added docs/workstation/world-class-sourceos-parity-plan.md.
• Integrated the latest Mac-on-Linux, macOS parity, and world-class SourceOS gap analysis.
• Captured current landed workstation-v0 surfaces, design misses, repo boundaries, maturity scorecard, and phased execution plan.

Validation evidence:

• test -s docs/workstation/world-class-sourceos-parity-plan.md
• grep -F "sourceos-shell already exists" docs/workstation/world-class-sourceos-parity-plan.md
• grep -F "Mac-like GNOME workstation v0" docs/workstation/world-class-sourceos-parity-plan.md
• grep -F "world-class OS" docs/workstation/world-class-sourceos-parity-plan.md

Known gaps:

• This is a planning/specification document only.
• It does not modify implementation scripts or workflows.

Non-goals preserved:

• No production readiness claim.
• No full macOS parity claim.
• No runtime behavior changes.