[APIView] POC Cross-Language Discussions & Proposals for APIView

Add a dedicated cross-language discussions experience to APIView that enables language architects and SDK teams to collaborate on API surface alignment across languages — with a structured proposal and voting workflow.

## Problem

Today, when reviewing APIs that span multiple languages (e.g., a TypeSpec-defined service), reviewers leave comments on individual language revisions with no structured way to:
- See all cross-language feedback in one place
- Propose changes to the shared TypeSpec definition
- Vote on proposals per-language team
- Track which cross-language discussions need attention or have TODOs

Start by reading the APIView experience and the existing language-revision review flow. Define what a dedicated cross-language discussions experience should cover, including shared TypeSpec proposals, per-language voting, and tracking discussions that need attention or TODOs; done means these requirements are addressed in a reviewed POC.

Written by the indexing model from the issue text.