Build structural command projections - #84

Summary

• add the library-owned traversal that emits every simple command exactly once
• preserve the legacy Clause projection with identical Clause instances and ordering
• carry deterministic occurrence roles and ancestry, including nested and iterator substitutions
• validate parser-owned trees, facts, spans, domains, redirects, identity reuse, and depth limits all-or-nothing

Verification

• rebased patch has the exact adversarially reviewed stable patch ID
• adversarial implementation review: GO
• dotnet build -c Release: 0 warnings/errors
• dotnet test -c Release: 1,162 passed
• 21 focused projection tests passed
• header and formatting verification passed
• strict OpenSpec validation passed
• Slopwatch changed-file scan: 0 issues