Add v0.3 structural analysis API - #83

Summary

• add the locked v0.3 structural syntax and analysis result types without breaking the v0.2 Clause surface
• model nested command substitutions as parent-owned SimpleCommandSyntax children
• lock deterministic execution order, ancestry, working-directory behavior, and fail-closed forms for Bash and PowerShell
• expand the design corpus for multiple, nested, iterator, stateful, literal, escaped, and unsupported substitution forms
• record the late-stage Web sample and safe Mermaid rendering work

Verification

• exact stable patch IDs preserved across the clean rebase
• adversarial design review: GO
• dotnet build -c Release: 0 warnings/errors
• dotnet test -c Release: 1,141 passed
• header and formatting verification passed
• strict OpenSpec validation passed
• Slopwatch changed-file scan: 0 issues

Scope

This PR establishes the public structural contract and design corpus. Parser population and executable projection are delivered in subsequent isolated slices.