Align generator roadmap contract docs

[cwinland]

Summary

• align the roadmap summary and detailed generator roadmap with the settled generator contract decisions
• document the package shape, opt-in model, runtime fallback, MVP boundary, and Define generator-targeted graph metadata hooks and constructor-selection contract #125 constructor-contract boundary
• add the Define generator-targeted graph metadata hooks and constructor-selection contract #125 closure parity subsection and sync the phase ordering with the current issue stack

Issue alignment

• Closes Define generator-targeted graph metadata hooks and constructor-selection contract #125
• Keeps Coordinate runtime prerequisites for generator-targeted output #121 as the umbrella tracker
• Leaves implementation issues such as Implement compile-time test graph and harness generation MVP #122, Generate provider-first tests from existing services and supported classes #123, Analyzer should suggest generating tests for supported untested code and required FastMoq packages #124, Implement generated scenario and suite scaffolding after graph and harness MVP #136, and Implement generator-backed framework-helper builders for repeated test patterns #137 open

Scope

• docs only
• no source code changes